A COMPARISON OF SURFACE PREPARATION FOR COATINGS BY
WATER JETTING AND ABRASIVE BLASTING
Lydia M. Frenzel, Ph. D.
San Marcos, Texas, U.S.A.
Prepared and Presented at 1999 WJTA Conference; Houston TX. Printed in the Proceedings.
Water Jetting, with and without abrasive addition, continues to impact the maintenance industry
and displace some of the traditional areas of dry abrasive blasting. This presentation focuses on
the similarities and differences in the visual reference photographs which are used in the global
1. INTRODUCTION AND BACKGROUND
Since 1985, the continued improvement in nozzles, seals, and pumps make it possible for reliable
removal of coatings and rust. Water jetting and wet abrasive blasting methods have gone from a
rare oddity to acceptance by the marine industry and becoming the preferred process for removal
of lead based paint or asbestos. The Technology Publishing Company's (TPC) annual survey of
painting contractors indicates that fifty percent (50%) of the identified painting contractors use
some type of high pressure or ultra-high pressure waterjetting or wet abrasive process. One
individual contracting firm says they have cleaned over 20 million square feet of surface.
Coatings manufacturers, notably International Paint Co. (Akzo Nobel), Hempel's Paints, and
Jotun have produced videos, technical product literature, and visual reference photographs to
train their technical representatives and clients. Ameron, Bridge-Cote of Canada, Devoe, Euro-
Navy, Sigma, W&J Leigh & Co., Watson Coatings, and Wasser Hi-Tech Coatings are additional
coating manufacturing companies who actively embrace the use of water in surface preparation.
Organizations such as SSPC ( Society for Protective Coatings), NACE Int. ( National
Association of Corrosion Engineers), and ISO ( International Standards Organization) provide
the grounds for consensus documents, that is, documents which are defined as a “general
agreement” or a “majority of opinion.” Consensus recommended practices and technology
updates provide a Common Language to describe problems. Every industry tends to define the
same problems in different terms. Adopting consensus language saves time and money.
Environmental concerns are driving movement to include water. Water Jetting and Wet
Abrasive Blast Cleaning are displacing traditional abrasive blasting in certain areas. Adversarial
points of views exist within the coatings industry. It took ten years to build a consensus and
issue the first standard on the use of high pressure waterjetting jointly by NACE and SSPC in
1995.3 NACE and SSPC have issued two other documents since that time.2,11 In 1998, ISO also
started work on a separate water jetting standard as they did not know the extent of the American
Over the years, European and American philosophies drifted apart in the adoption of consensus
language for abrasive blasted cleaned steel. In simplistic terms, the Americans use standards
language allowing a percentage coverage of stains, while the Europeans use a criteria of tightly
adherent material. The visual reference photographs for abrasive blast cleaning showed
examples of rusted steel, but not removal of coatings. Projects involving removal of coatings for
repainting often call for leaving sound, adherent coatings on the substrate, not removing all of it
to leave slight stains. These pictures don't exist as consensus photographs. The development of
photos for high pressure water jetting is bringing the European and American working
philosophies together as we consider the maintenance practices on a global basis.
In the fall of 1998, Dr. Frenzel drafted a letter which was sent to ISO over Ken Tator's, as the
designated U.S. country expert to ISO TC 35 SC 12, signature with full support of SSPC and
" Dr. Lydia Frenzel, chair of the NACE and SSPC Joint Task Groups on water jetting and wet
abrasive blast cleaning (TG D and TG I), and I would like to establish communication on a
regular basis between the ISO working groups on surface preparation concerning water jetting
and the Joint Task Groups in a mutually beneficial manner. Dr. Frenzel and I meet on a regular
basis. We are concerned that the ISO groups may not fully aware of standards activities in the
North America and wish to avoid conflict between working groups. We hope there will be
commonality in standards development between the North American and the European
We recognize that an independent set of photographs should be prepared so that coatings
manufacturers do not have to refer customers to material originally prepared by a competitor.
The U.S. National Shipbuilding Research Program funded the preparation of a set of
photographs of surfaces prepared by water jetting specifically for standards preparation. These
new photographs have been released to SSPC and NACE for preparation of visual reference
.....[we] hope that we can work together and provide a commonality to the water jetting and wet
Following this letter, ISO had a representative to the SSPC/NACE Task Group meeting in
November, 1998, and members of the NACE/SSPC Task Group and the SSPC Executive
Director, Dr. Bernard Appleman, met informally with ISO Working Group 2 on Water Jetting in
March, 1999. Documents and working photos were exchanged. We opened communications
and are currently working on development of new visual reference photographs for the Water
Jetting of substrates, in particular steel, with the Europeans. This is vitally necessary as coatings
suppliers and contractors work globally.
The majority of the slides used in this presentation are proprietary to individual companies or are
actual slides of the draft photos under consideration by the standards groups. As such, they are
not reproduced in this paper.
2. DEFINITIONS USED BY COATINGS INDUSTRY
The paper will be using definitions from SSPC and NACE technical reports and standards which
are closely aligned with the WJTA Recommended Practices for the Use of Manually Operated
High Pressure Water Jetting Equipment.1,2,3 The SSPC and NACE documents are used by the
coatings professionals. In these documents, the distinction is made that blast cleaning or blasting
involves the use of solid abrasives whereas water cleaning or water jetting is the use of water
alone without abrasives. "Water Blasting" is such a generic and wide-spread term that it hasn't
been defined in the consensus process.
Wet Abrasive Blast Cleaning (WAB) covers procedures, equipment, and materials involved in a
variety of air/water/abrasive, water/abrasive, and water-pressurized abrasive blast cleaning
systems. Air/water/abrasive blasting is the specific cleaning method in which water is injected
into the air/abrasive stream generated by conventional air-pressurized abrasive blasting
equipment. Water/abrasive blasting is a cleaning method in which abrasive is injected into the
water stream generated by conventional fluid pumps.
Other generic terms to describe specific air/water/abrasive blast cleaning methods are: Water
Shroud or Wet-Head blasting, wet blasting, low volume water abrasive blasting, and slurry
blasting. Other generic terms to describe specific water/abrasive blast cleaning methods are:
slurry blasting, abrasive water jet (AWJ), or abrasive injected water jetting/blasting (AIWJ).
High Pressure Water Jetting (HP WJ) is cleaning performed at pressures from 70 to 216 MPa
(10,000 to 30,000 psi). Ultrahigh Pressure Water Jetting (UHP WJ): cleaning performed at
pressures above 216 MPa (,000 psi). Low Pressure Water Cleaning (LP WC) is cleaning
performed at pressures less than 34 MPa (5,000 psi) High Pressure Water Cleaning (HP WC) is
cleaning performed at pressures from 34 to 70 MPa (5,000 to 10,000 psi).
The terms hydroblasting, hydrojetting, water blasting, and water jetting describe the process in
which pressurized water is directed through a nozzle to impact a surface. However, it is noted
that the terms hydroblasting or water blasting is used generically to describe cleaning methods
that range from low pressure water cleaning to ultrahigh pressure water jetting.
In the coatings industry, water jetting does not provide the primary anchor pattern. The use of
water alone is primarily for recoating or relining projects for which there is an adequate,
preexisting profile. Abrasive and water combinations can be used on older substrates or new
projects to establish a new profile or anchor pattern
3. COATINGS FAILURES AND SURFACE PREPARATION
In the world of corrosion control and painting, it should be obvious to everyone that the job is to
get the surface clean enough to accept the paint system. This process is called surface
preparation. Surface Preparation is- creating the situation so that the coatings will perform
as expected. Remarks in this paper will be limited to processing metal substrates rather than
wood, concrete, or plastic and will not include chemical processes such as etching or
If you don’t produce a clean surface so that the paint will adhere, the world will move elsewhere.
DO a GOOD JOB, the world will beat a path to your door. Expectations for the quality of
surface preparation in coatings have escalated in the past few years. Change increases
exponentially. This is the trend of the future. Preparing for change is preparing for the future.
Using water in surface preparation, with and without abrasive, is part of the future. Think of the
future of your business. If you adapt what you see and hear to your particular needs, you will be
part of the future.
It is frequently said that ninety percent (90%) of all coatings failures are the fault of the surface
preparation. It is also said that "seventy-five percent (75%) of all coating failures are the fault of
the contractor." There are many factors that influence the performance or lifetime of a coating
system in addition to the surface preparation, such as formulation, application, and service
conditions. Lou Vincent examined failure modes of protective coatings in a presentation to
SSPC in November, 1998 and identified twenty-two types.4 Three of those twenty-two failure
modes are directly related to surface preparation- adhesion loss, blisters, and delamination.
Those three failure modes accounted for 58% of the failures in 55 field case occurrences and
46% of the failures in 101 literature articles occurrences. While this is not the 75-90% generally
cited, adhesion loss, blisters, and delamination are clearly the primary failure modes. The other
40-50% of the failures are spread between nineteen different types of failures.
The use of water in maintenance applications, not new steel construction, can have a very
positive increase of the adhesion of the coatings and can reduce the chance for blisters and
delamination. This positive benefit is why the use of water is the future evolution of surface
4. THREE ELEMENTS FOR A SUCCESSFUL PROJECT
People are driven to include water by environmental, safety, and economic considerations. They
are not embracing water for the benefit of enhanced performance. To make water work in a
project, you have to understand that THREE viewpoints must converge in bid specifications and
they must all be represented in the negotiations and planning of a project.
The viewpoints of the
and coatings manufacturer
must all come together.
This may seem obvious but it is an often neglected principle. The guarantee for a good job is
forced on the coatings manufacturer and contractor. The contractor may be prevented from
using wet abrasive blasting (WAB)2 or water jetting (WJ)3 even though the coatings
manufacturer and contractor both agree that WAB or WJ may be preferable to clean a surface if
the client or owner only has knowledge or training in dry abrasive blast cleaning. Why?-
because "the customer (in this case, the owner) is always right." If the customer only has
experience in dry abrasive blasting, then the contractor and coating manufacturers working
together must overcome steep opposition and provide education.
People in the coatings and maintenance industry are afraid of change. There is a lot of resistance
towards change. It is easy to understand. Mistakes come back to haunt us. As an industry, we
are to be blamed for slow acceptance-because we don’t talk to each other. Everybody is
jealously guarding their secrets. If an engineer learns something that will give him lower
maintenance costs or an edge on the competition, he doesn't like to share that secret. The goal of
zero defects on every job is a slow, ongoing process. This presentation is part of education.
Education is the key to understanding.
5. THREE COMPONENTS OF SURFACE PREPARATION
Surface Preparation- creating the situation so that the coatings will perform as expected.
There are three components to Surface Preparation - All are necessary even though the emphasis
in the past has been only on visible cleanliness and anchor profile.
Everyone in the coating industry is trained on the visible requirements from day one. The
coatings manufacturers control the anchor profile requirements. The third component, Invisible
Contaminants, is one component that people are still unaware of even after at least fifteen years
of education. All three components are all equally important. While all three are necessary for
good coatings performance, it is the last component, the invisible contaminants, that demands
water and which requires reform.
The addition of water in surface preparation evolution is occurring because coatings
manufacturers have recognized the secret and success of water in dealing with invisible salts.
The coatings manufacturers have really understood that water is the "True Grit of the 21st
Century." The coatings manufacturers have come forward with videos, pictures, and have
rewritten their specifications so that surfaces can be cleaned with dry abrasive blasting and/or
various water/abrasive methods.
One major obstacle of the acceptance of water in surface preparation is the appearance. Another
major obstacle is the formation of flash rust. Part of the maintenance industry will never accept
the appearance of the surface when water is used in surface.
5.1 ANCHOR PROFILE
5.1.1 NEW METAL
First how does one create the profile on a new piece of metal? The profile or anchor pattern is
specified by the coating manufacturer. In simplistic terms, the profile of the substrate is
generally considered to be the dominant factor in coating adhesion.
Water Jetting by itself is generally not used to create the initial profile even though creating a
surface profile can be accomplished with water jetting alone on small objects with careful
controls. 5,6 Automated equipment must be used to control the depth, transverse rate, and stand-
off distance. The process is too slow for large pieces. However, when the profile is produced by
water alone, the adhesion is greatly enhanced.5
The major surface profile on a metal substrate is defined by the abrasive and is typically formed
by a dry, abrasive blast technique. Because of environmental restraints on visible dust, Wet
Abrasive Blasting (WAB) is finding a market in new steel construction. WAB covers techniques
which range from mostly abrasive, mixed with a little water to suppress the dust, to mostly water
with a little abrasive. The pressure range of the water flow can be anywhere from 50 psi to
The anchor profile, or pattern, on the substrate is specified by the coatings manufacturer to a
depth, such as 0.002-0.003 inches (50-75 microns). Rounded particles such as steel shot give a
rounded, crater-like appearance where the width of the depression is greater than the depth. It is
thought that hardened sharp angular abrasives tend to cut into the metal, leaving sharp edges,
creating hackles (small slivers of steel standing perpendicular to the surface). Particles moving
faster will make a deeper indentation compared to the same particle moving slower.
An analogy is throwing a baseball at a mud flat. Throw a large softball relatively slow and you
will make a rounded impression with a lip. Throw a small hard ball relatively fast and you will
make a deeper rounded impression. Throw a sharpened pyramid at an angle, and you will create
a ridge. Some of the mud will splatter off, but most of the mud gets shifted to the new profile.
Abrasives do not necessarily remove the metal because metal is malleable, but you are creating
a macroscopic pattern. This pattern may or may not remove existing corrosion cells.7 This
initial profile provides the cleaned nib to which the paint adheres.
5.1.2 OLD METAL
There are many situations where abrasives are needed when older surfaces are being blasted.
Abrasives are used in tight corners and for the back side of plates where the particles can be
rebound or ricochet. In marine areas, there is frequently a very tightly adherent black layer of
rust which is resistant to removal by water jetting alone. Addition of a little abrasive into the
water stream will speed the production rate and help break this brittle layer. Abrasives can be
used to ricochet on all sides of a small compartment whereas it may be difficult to direct a jet
stream of water towards all the surfaces.
Abrasives change the existing profile. Abrasives erodes or abrades the surface from the top. A
US Standard 100 mesh screen has openings for 125 micron particles. When the contractor is
cleaning with 125 micron particles, contaminants which are in cracks or crevices or pits simply
cannot be reached or removed. Crevices become filled with spent abrasive when a contractor is
trying to remove pack rust between plates. Subsequently the paint gets applied over "clean"
abrasive and fails prematurely. If there are invisible contaminants on the surface, abrasive
blasting can drive the contaminants into the surface or form a pocket of metal in which the
contaminant is buried.
Waterjetting can be described as a series of small droplets in the 5-10 micron range hitting the
surface at supersonic speeds. The droplets implode (cavitate) and drill through coatings or rust;
then spread laterally and shear at the interface, much like ultrasonic cleaning, to lift materials. A
series of microscopic “pock marks” form on the macro surface. The craters and pits get "deep
The measured profile in gross terms, for example 0.002inch (50 microns), is still the same for a
surface cleaned by abrasive and by water jetting, but the microscopic details are different. The
amount of surface area per square unit area is increased for the WJ cleaned surface. Figure 1 and
Abrasive blasting changes the surface from the top down; water cleans the surface from
the bottom of the pits up. These two different types of actions lead to two distinct visual
appearances. Waterjetting cleans the existing profile and opens it. Abrasive blasting
ignores the pits and cracks. There is a synergistic effect in using abrasives and water
because you can get the advantage of both processes.
In a direct comparison of UHP WJ with abrasive blasting, Materials Evaluation Laboratories
concluded “The pressurized water method was considered the best preparatory cleaning for non-
destructive inspection. It offered a more authentic representation of the surface than the other
methods evaluated….Pressured water had minimal disturbance of micro-structural features. ”8
In a direct comparison of water blasting at 10,000 psi versus abrasive blasting for penstock
relining,9 Tom Aldinger reported that water blasting would give as good or better adhesion than
abrasive blast to SSPC SP10 (near-white) on 60 year-old pen stock. Water blasting produced a
clean surface without rust and loose paint residues. Atlas Cell testing in deionized water at 140
Deg. F was then used to compare immersion service performance for coatings on the water blast
and abrasive blast surfaces. At the end of the Atlas Cell test, the investigators found a thin film of
water and black rust under both the urethane and epoxy coatings on the abrasive blast surface
with complete adhesion failure in the immersion zone. The water blast surface had coating
adhesion 3 times greater compared to the coating on the abrasive blast surface. Any substrate
corrosion on the water blast surface was local without any underfilm spreading. Underfilm
corrosion has also been noted in cases where salt was deliberately added to the surface even
though the paint was applied before any rust was present.10
“Adhesion begins at the bottom of the pits” said James Denny, Vice-President of International
Coatings at Corrosion96. Water Jetting cleans the bottom of the pits. It is the experience of
International Coatings and the marine industry that coatings adhere better and last longer on
surfaces which have been cleaned by Water Jetting. Coatings manufacturers understand that
when you use water for cleaning a profile you get better adhesion sometimes as much as two
fold. Van Kuiken's patent illustrates this point. The micro profile is fractal for waterjetting. Loss
of adhesion as a failure mode disappears.
5.2 INVISIBLE CONTAMINANTS
Invisible contaminants such as oil and grease generally lead to delamination as a coating failure.
Delamination also can be caused by a minimal substrate profile. Invisible contaminants such as
salts, chemicals, or water soluble substances lead to osmotic blistering as a coating failure mode.
The removal of the invisible contaminants leads to longer performance by the coating system.
The ability to remove chemical contaminants (salts), particularly from badly pitted and corroded
steel, is a major advantage of the water jetting process.11,12
WJ and WAB do such a good job of removing invisible contaminants from the surface, even if
intact coatings are left on, that blisters from chemical contaminants and delamination from oil
and grease disappear.
5.3 VISIBLE APPEARANCE
If WJ and WAB are so good at surface preparation, why is there a resistance for its adoption?
Surfaces cleaned by water alone do not look like surfaces modeled by abrasives. Flash rusting on
a steel surface can occur very quickly as a result of the very fine, sharp edges. Most of the
resistance is based on the visible appearance.
Contractors, inspectors, and coatings personnel use the SSPC/NACE and ISO written standards
and visual reference photographs for training and acceptance on jobs.
The written standards include:
NACE NO. 5- SSPC SP-12 for water jetting
SSPC SP-5 for abrasive blasting
SSPC SP-10 for abrasive blasting
SSPC SP-6 for abrasive blasting
SSPC SP-7 for abrasive blasting
Visual reference Photographs include:
ISO 8501-1 for dry abrasive, hand-tool or power-tool cleaning, flame cleaning
SSPC VIS-1 for dry abrasive blast cleaning
SSPC VIS-3 for hand and power tool cleaning- This shows removal of coatings.
International Paint For Water Jetting issued in 1994
International Paint for Slurry Blasting (Wet Abrasive Blast cleaning)
Hempel's Photo Reference for Steel Surfaces cleaned by Water Jetting
Jotun Photo Reference for examples of flash rusting
Schiffbautechnisch Gesellschaft No. 2222 Guide for water jetting
SSPC- VIS 4 (I) NACE No. 7 Interim Guide and Visual Reference Photographs for Steel
Cleaned by Water Jetting- issued in 1998
Even though the visible references are for supplemental purposes in the US, in practicality,
people use them as a primary standard. SSPC VIS-1 and ISO 8501-1 are the two visual
reference photographs series used in training. They depict dry abrasive cleaned steel which have
not been painted. Emphasis is on uniformity. Inspectors and owners are just beginning to use the
VIS-4 (I) Reference Photographs for Water Jetting ( which is the same set of photos as
International Paint Water Jetting Standards. These also only depict unpainted steel.
As we go through the standards, keep in mind that visual reference photographs are designed to
be illustrative of the situation. Direct correlation to existing dry media blasting standards is
inaccurate or inappropriate when describing the capabilities of waterjetting and the result
achieved with waterjetting as a process.
Abrasives hit from the top, erode the surface, provide plastic flow to the metals, tend to make the
surface look uniform and “erase” different areas. There is a tendency to drive existing
contaminants into the surface. Observers with an experienced eye tend to neglect the pits. They
tend to look at the top surface.
WJ stresses the adhesion between two materials. WJ retains the metallic surface profile, tends
to clean the pits first and leave material at the top peaks, and accentuate the non-uniformity of a
surface. The experienced observer sees black stains on the top where heavy rust was present, or
coatings on the top of the surface- rather than stains in the bottom of the pits so the observer see
something which is a new experience even though the pits are cleaned.
The visual photographs SSPC VIS-1 and ISO 8501 only depict the situations of rusted steel.
Water jetting is used primarily in removal of coatings, where frequently the objective is to retain
as much tightly adherent coating as possible. There was a need to address the question of
maintenance in visual reference photographs.
The following phrase exists in all the written standards. “Acceptable variations in appearance
that do not affect surface cleanliness include variations caused by type of steel, original surface
condition, thickness of the steel, weld metal, mill or fabrication marks, heat treating, heat
affected zones, blasting abrasive, and difference in the blast pattern.” Figures 3 and 4 illustrate
WJ-1 cleaning and the variation in appearance. That variation does not appear in the abrasive
5.3.1 CLEAN TO BARE SUBSTRATE- "WHITE METAL"
NACE No. 5- SSPC - SP 12 WJ-1
WJ-1 surface shall be free of all previously existing visible rust, coatings, mill scale, and foreign
matter and have a matte metal finish.
ISO 8501-1 Sa 3
When viewed without magnification, the surface shall be free from visible oil, grease and dirt,
and shall be free from mill scale, rust, paint coatings and foreign matter. It shall have a uniform
SSPC-SP 5 NACE 1
When viewed without magnification, the surface shall be free of all visible oil, grease, dust, dirt,
mill scale, rust, paint, oxides, corrosion products, and other foreign matter.
5.3.2 VERY THOROUGH CLEANING, "CLEAN ALMOST TO BARE SUBSTRATE"
NACE No. 5- SSPC SP-12 WJ-2
WJ-2 surface shall be cleaned to a matte finish with at least 95 percent of the surface area free of
all previously existing visible residues and the remaining 5 percent containing only randomly
dispersed stains of rust, coatings, and foreign matter.
SSPC SP 10 NACE No. 2 “Near White Blast Cleaned Surface”
RANDOM staining shall be limited to no more than 5 percent of each unit area of surface ... ,
and may consist of light shadows, slight streaks, or minor discoloration caused by stains of rust,
stains of mill scale, or stains of previously applied paint.
ISO 8501-1 Sa 2 1/2 Very Thorough Blast Cleaning:
When viewed without magnification, the surface shall be free from visible oil, grease and dirt,
and from mill scale, rust, paint coatings and foreign matter. Any remaining traces of
contamination shall show only as slight stains in the form of spots or stripes.
HB 2.5 Very Thorough Hydroblast Cleaning (International Paint)
When viewed without magnification, the surface shall be free from visible oil, grease, dirt, loose
rust, paint coatings and foreign matter. A brown-black discoloration of ferric oxide may remain
as a lightly adherent thin film on corroded and pitted steel.
In conversations with major coatings manufacturers, Dr. Frenzel has come to understand that the
coating manufacturers' technical staff are not including the brown-black discoloration of ferric
oxide as part of the staining criteria. They are looking for stains of material other than black
ferric oxide. Thus a WJ-1 or WJ-2 may be extremely mottled if the steel surface has been
5.3.3 THOROUGH CLEANING, “COMMERCIAL BLAST”
NACE No. 5- SSPC SP-12 WJ-3
WJ-3 surface shall be cleaned to a matte finish with at least two-thirds of the surface free of all
visible residues (except mill scale), and the remaining one-third containing only randomly
dispersed staining of previously existing rust, coatings, and foreign matter.
Notice that we are now including the idea that mill scale might remain on the surface. This is
recognition of the types of projects in which WJ is used. The language is for staining, not for the
coating itself. However, the marine industry is interpreting this to mean the coatings and foreign
matter can remain on if it is dispersed.
SSPC SP- 6, NACE No. 3 Commercial Blast
Random staining shall be limited to no more than 33 percent of each unit area of surface as
defined, and may consist of light shadows, slight streaks, or minor discoloration caused by stains
of rust, stains of mill scale, or stains of previously applied paint.
ISO 8501 Sa 2 Thorough Blast- Cleaning
When viewed without magnification, the surface shall be free from visible oil, grease and dirt,
and from most of the mill scale, rust, paint coatings and foreign matter. Any residual
contamination shall be firmly adhering.
HB 2 Thorough Hydroblast Cleaning (International Paint)
When viewed without magnification, the surface shall be free from visible oil, grease, dirt and
from most of the rust, paint coatings and foreign matter. Any remaining contamination and
staining shall be firmly adherent.
Tightly adherent coatings, mill scale and rust can remain in the ISO definition, but not in the
5.3.4 BRUSH OFF BLAST, REMOVAL OF LOOSE MATERIAL
NACE No. 5/ SSPC SP-12 WJ - 4
WJ-4 surface shall have all loose rust, loose mill scale, and loose coatings uniformly removed.
SSPC SP- 7 Brush Off Blast
Tightly adherent mill scale, rust, and paint may remain on the surface. Mill scale, rust, and paint
are considered tightly adherent if they cannot be removed by lifting with a dull putty knife.
ISO 8501 Sa 1 Light Blast Cleaning
When viewed without magnification, the surface shall be free from visible oil, grease, and dirt,
and from poorly adhering mill scale, rust, paint coatings and foreign matter.
For WJ-1 "Clean to bare substrate" and WJ-2 "very thorough cleaning" specifications, the
WAB and WJ appearance is dark compared to dry blasting. It may also be streaked from the
drying of the water. This appearance is very normal and accepted by the coatings manufacturers.
On older steel, heat marks, tooling, and dark stains in corrosion areas remain very visible with
WJ. WJ accentuates those differences; abrasive blasting tends to make the surface "appear"
When VIS-4(I) was issued, the NACE/SSPC task group modified the language to accommodate
both the adherent (ISO) and the percentage (NACE-SSPC) concepts and started to bring the
European and United States philosophies closer together. Because of obstacles arising in the
field in refurbishment projects, there was an immediate need for photographs depicting coatings
removal. In January, 1997, after a review and selection of new photographs, the National
Shipbuilding Research Program SP-3 Technical Advisory Chairman for Water Jetting
Photographs requested an early revision of NACE No.5- SSPC SP-12 so that the language might
reflect the retention of coatings. It was obvious from the selection of the photographs and the
issuance of the Hempel's reference set that there was a wide gap between the language and the
practice. In all of these cases, technical representatives of coatings manufacturers, owners, and
experienced WJ contractors were involved.
The perception of percentage coverage is an unrecognized problem. Every experienced
inspector thinks that he can tell what 5% and 33% coverage is. That simply is not true. This was
very evident as the new photos were being screened.
Coverage is a topic to itself and is depicted in illustrations of 5% and 30% coverage from "The
Book of Spots."13 Each person views a surface differently. Whether the spots are sharp or
diffuse, nearly the same color or contrasting colors, on a uniform, lightly profiled or a non-
uniform, heavy pitted surface, will make a difference. Since 1996, Dr. Frenzel has been
educating the experienced personnel first on the NSRP Technical Advisory Committee and
subsequently the NACE/SSPC task group on percentage coverage. Figures 5, 6, and 7 illustrate
three different representations of five percentage coverage.
In practice, the marine coatings industry embraced NACE No. 5/ SSPS SP-12 and immediately
interpreted it to allow islands of tightly adherent paint to remain as compared to stains of paint
because it is used in maintenance and repainting. The original "staining" language arose
because solid particles don't get into the crevices. The paint remains as stains in the pits and
crevices. In WJ without abrasive, intact coating remains on the top of the surface. The new
photos under consideration where coatings are being removed all depict the retention of coatings.
Coatings manufacturers have positively endorsed this position.
With the advent of computer digitization, new photos are also being considered for SSPC- VIS
2/ASTM D 610 "Standard Method of Evaluating Degree of Rusting On painted Steel Surfaces."
The concept of percentages with large, medium, and small, pinpoint spots is a challenge because
the appearance in pin point corrosion at 30% is that the entire surface is covered.
The written definitions are similar in that they describe four visual cleanliness conditions. They
differ with respect to the presence of mill scale and tightly adherent coating as compared to
percentage staining. Based on the discussion of the task group members, it is the author's
opinion that the current ISO photographs and the SSPC/NACE written definitions are inadequate
to address the problems of retention of sound coatings in maintenance. The appearance of old
metal surfaces cleaned by WJ without any abrasive is very different from those cleaned with
There is no discrepancy when all coatings and rust layers are removed by WJ or WAB.
However, WJ finds its forte in partial removal of coatings. It is the partial removal and spot
blasting with WJ and WAB with soft abrasives that is forcing the adoption of new visual
photographs and a revision of the written standards language. The WJ task groups of NACE,
SSPC, and ISO are addressing this question. Water jet cleaning is bringing the European and
United States standards organizations together into a coalition effort.
Thanks to International Paint, Jotun, Hempel's, Cavi-Tech Inc. for permission to use their
photographs in non-commercial presentations. The members of the SSPC and NACE task groups
have volunteered thousands of hours. Special thanks to Aqua-Dyne, Butterworth Jetting,
Carolina Equipment & Supply, Flow Int., and NLB Corporation, Doug & June Koppang,
Aulson Co., Cavi-Tech, Fluidyne, Freemyer Co., Hartman-Walsh Painting, UHP Projects,
Valley Systems,, Leo Kosowan, Roland Hernandez, and Dan Bernard for support and discussion.
Figure 1. Steel Cleaned with HP WJ Figure 2 Steel Cleaned with Abrasive
130x magnification. Upper right white 130x magnification. Upper right white
bar is 10 microns. originally blasted, then bar is 10 microns. Originally blasted,
rusted, then cleaned with HP WJ to then rusted, then blasted to white metal.
“white metal” WJ-1. You can see the You can see the impacts, the flattened
original impacts of the abrasive. All surface. The dark areas are materials
crevices are cleaned; the dark areas are caught in the lower layer under the top
Figure 3 Multilayer Paint cleaned with WJ Figure 4 WJ-1 "Clean to bare substrate"
Original size 6 x 10 inches Upper right and left show typical
partial removal of paint. Complete removal examples of carbon stain and the
on lower edge. appearance of corroded steel which is
cleaned to WJ-1. The steel under the
paint is uniform and appears "white" in
this black and white printing.
Figure 5 Five Percentage coverage- Figure 6 Five percentage coverage-
Larger Spots in a Distribution of 1:10 Smaller spots in a distribution of 1:10
Figure 7 Five percent coverage-
Uniform distribution of Very Small Dots.
“Recommended Practices for the Use of Manually Operated High Pressure Water Jetting
Equipment,” WJTA, St. Louis MO
SSPC-TR2/ NACE 6G198, SSPC/NACE Joint Technical Report, "Wet Abrasive Blast
Cleaning," issued May, 1998. SSPC Tel: 412-281-2331
Joint Surface Preparation Standard NACE No. 5/SSPC-SP 12; "Surface Preparation and
Cleaning of Steel and Other Hard Materials by High- and Ultrahigh-Pressure Water
Jetting Prior to Recoating,", issued 1995. NACE Tel: 281-228-6200 Separate document
"Surface Preparation of Concrete" includes WJ cleaning of concrete.
Vincent, L.D. , "Increasing the Value of Coatings", Proceedings of the SSPC 1998 Seminars,
November, 1998, SSPC 98-11 "Failure Modes of Protective Coatings and Their Effect on
Management," p. 125-128.
VanKuiken, Jr., L.L., Byrnes, L.E., & Kramer, M.S., High Pressure Water Jet Method of
Blasting Low Density Metallic Surfaces, U.S. Patent 5,380,564, Issued Jan. 10, 1995
Taylor, Thomas A., "Surface Roughening of Metallic Substrates by High Pressure Waterjet,"
Surface & Coatings Technology, Vol. 76-77 ( 1995), 95-100
Frenzel, Lydia M.; Nixon Jonell, "Surface Preparation using High Pressure Water Blasting,"
NACE Corrosion89, paper No. 397, April, 1989
Materials Evaluation Laboratories, Claude Mount, January 1991, work performed for Shell Oil
Company, Report number 9501
Aldinger, Tom (Bechtel), Viswanath, Bala (Pacific Gas and Electric), Dick Vass (Vass
Industries), SSPC 1994 Conference, Industrial Maintenance Coatings- Current Trends
and Practices, “Water Blasting versus Abrasive Blasting for In situ Penstock Relining”
Morcillo, M. & Simancas, J., JPCL, Sept. 1997, p. 40 “effects of Soluble Salts on Coating Life
in Atmospheric Service" and G.C. Soltz., “The Effect of Substrate Contaminants on the
Life of Epoxy Coatings Submerged in Sea Water,” National Shipbuilding Research
Program, March, 1991, task 3-89-2
SSPC-VIS 4(I) NACE No. 7 "Interim Guide and Visual Reference Photographs for Steel
Cleaned by Water Jetting, SSPC Pub. 98-07; NACE Item NO. 22016
Howlett, Jr., J.J. and Dupuy, R., Ultrahigh-Pressure Water Jetting for Deposit Removal and
Surface Preparation, Materials Performance (MP), No.1, Jan, 1993, p. 38
Advisory Council, "The Book of Spots", depicting 1-30% coverage and 20% with different
relative spot sizes. Computer generated to be exact.