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Wet, Dry, Other: A Surface Prep Primer


By Lee Wilson

I have written many technical papers and blogged on numerous aspects of surface preparation from standards to new advancements in abrasive media, and still I have yet to scratch the surface (pardon the pun) when it comes to the vast range of surface prep sciences, technologies and methods used within the coating industry.

It is widely held that 60 to 85 percent of all premature coating failures can be attributed directly to inadequate surface preparation, regardless of whether you are applying the latest and greatest protective coating system. If you have not cleaned the substrate, you can expect premature failure of your protective coating or at the very least a reduced service life. Therefore, you can see the importance of surface preparation and the endless debates about it that can ensue when it comes to selecting the appropriate method of surface preparation for your project requirements.

There is a vast range of surface preparation methods widely used within the industry today for pre-treatment of ferrous-based materials. These include methods such as:

  • Solvent cleaning;
  • Hand tool cleaning;
  • Power tool cleaning;
  • Dry abrasive blasting;
  • Wet abrasive blasting;
  • Open blasting;
  • Enclosed blasting;
  • Water jetting;
  • Acid pickling; and
  • Phosphating.

There are many other methods used as well, such as flame cleaning and bristle blasting.

Abrasive blasting
Technology Publishing Co.

Of the different types of surface preparation available, dry abrasive blasting is the most established and commonly used method for the cleaning of steel.

Most of the above highlighted methods of surface preparation come with their own standards, which explain acceptance and rejection criteria with regard to each. Institutes such as SSPC/NACE, ASTM and ISO have been instrumental in creating and revising standards for most methods of surface preparation.

Adhesion and Its Enemies

I addressed the importance of adhesion in a previous blog. However, nearly all aspects of surface preparation and subsequent inspection and testing are linked to adhesion, so both fields tend to go hand in hand. Strong adhesion is the key to coating performance and long life: Simply put, if adhesion is weak, then a coating will likely fail. Surface preparation is carried out in order to promote adhesion.

Typical examples of factors that greatly affect adhesion are:

  • Oil or grease;
  • Acids and alkalis;
  • Soluble salts;
  • Mill scale;
  • Chlorides and sulfates;
  • Old paint;
  • Rust;
  • Chemicals;
  • Fumes; and
  • Debris.

There are many more potential contaminants found on steel that could greatly affect the adhesion of any coating system, all of which need to be removed prior to application. This is where the surface preparation methods listed above come into play.

The initial steel condition plays a major role in determining the amount of time, work and effort required in order to achieve any particular degree of surface preparation. This is because it is necessary to take into consideration the amount of rust, old paint, contamination, and active corrosion or pitting on the surface to be protected. The greater the degree of contaminants, of course, the more care and attention is required in order to remove them.

We also must consider that a lot of the contaminants listed above will not only greatly affect the adhesion of any applied protective coating system, but are in themselves corrosion accelerators and can, under the right conditions, greatly increase the corrosion rates of steel.

It is the end result that is the most important factor, since the end result ultimately dictates the degree of contamination that remains on the surface after surface prep has been carried out, and therefore the degree of physical or mechanical adhesion.

Tried and True: Dry Abrasive Blasting

Of the different types of surface preparation available, dry abrasive blasting is the most established and commonly used method for the cleaning of steel. This is usually achieved by projecting a highly concentrated stream of abrasive under pressure (typically around 100 psi, or 7 bar) through a pressurized blasting system to the substrate to be prepared.

Abrasive blasting
© / GlenJ

Dry abrasive blasting not only removes mill scale and rust from a steel surface; it also creates a surface profile that promotes adhesion of the coating.

Dry abrasive blasting has the advantages of efficiently cleaning the steel substrate, subsequently removing mill scale, rust and paint from the surface and creating a surface profile or anchor pattern, which promotes adhesion of the subsequent protective coating system. This factor alone is of major importance to the lifespan of the coating system.

Although dry abrasive blasting is the most commonly used method of surface preparation it also has its disadvantages, and in some cases cannot be used as the preferred method of surface preparation. Some of the disadvantages include:

  • Abrasive and abrasive dust may damage sensitive equipment and instrumentation.
  • Dust may contaminate the surrounding or adjacent area.
  • Extensive safety equipment (including breathing apparatus) is required.
  • Habitats and enclosures are required in order to contain the abrasive and protect adjacent work operations and operators.
  • Some substrates cannot be blasted.
  • Spent abrasive needs to be removed.

One of the most prominent disadvantages found with dry abrasive blasting is that the blasting process can press contaminants into the surface, and the method will not remove soluble iron salts.

The most universal standard used within the industry today is the joint standard Near-White Blast Cleaned Surface (SSPC-SP10/NACE #2). This is a standard used to determine the surface cleanliness achieved by dry abrasive blasting. The standard states that the surface:

[…] when viewed without magnification, shall be free of all oil, grease, dust, dirt, mill scale, rust, coatings, oxides, corrosion products, and other foreign matter, except from staining which shall be limited to no more than 5% of each unit area of surface (9 in.2 / 6400 mm2), which may consist of light shadows, slight streaks, or minor discoloration caused by stains of rust, stains of mill scale or stains of previously applied coating.

The Rise of Water Jetting

The use of water as a method of surface preparation has been on the increase within the industry. In recent years, this has been predominantly due to the environmental advantages that the method offers, and the successful removal of contaminants both visual and non-visual that this method creates.

Water jetting has one great advantage over dry abrasive blasting in that it is a more efficient method of removing non-visual contaminants such as chlorides, nitrates and sulfides from a steel surface.

However, water jetting also has its disadvantages—for example, this method:

  • Does not remove mill scale;
  • Does not create a surface profile;
  • Consumes a great deal of water;
  • Creates poor visibility;
  • Comes at a relatively high cost;
  • Requires high operator thrust;
  • Induces pperator fatigue; and
  • Can’t be used around electrical equipment.

In 2002, SSPC/NACE jointly created the water jetting standard SSPC-SP12/NACE No. 5, Surface Preparation and Cleaning of Metals Prior to Re-coating. Like the dry abrasive blasting standards, this standard for water jetting is also accompanied by a full visual standard, SSPC-VIS 4/ NACE VIS 7, Guide and Reference Photographs of Steel Surface Preparation by Water Jetting.

Water jetting
© / firmafotografen

The use of water as a method of surface preparation has been on the increase within the industry.

This standard has since been replaced, in 2012, and the original standard has been subsequently withdrawn. I am led to believe that this was carried out in order to bring the water jetting standards into some kind of correlation with the dry abrasive blasting standards.

The standard was replaced with four new joint standards: SSPC SP WJ-1/NACE WJ-1, SSPC-SP WJ2/NACE WJ2, SSPC-SP WJ3/NACE WJ3 and SSPC-SP WJ4/NACE WJ4. There do not appear to be any great changes in respect to the designation of the required pressure levels from the preceding standard:

  • Low-pressure water cleaning (LP WC), performed at pressures less than 34 MPa (5,000 psig);
  • High-pressure water cleaning (HP WC), performed at pressures from 34 to 70 MPa (5,000 to 10,000 psig);
  • High-pressure water jetting (HP WJ) performed at pressures from 70 to 210 MPa (10,000 to 30,000 psig); and
  • Ultra high-pressure water jetting (UHP WJ) performed at pressures greater than 210 MPa (30,000 psig).

The different degrees of cleanliness required by WJ-1, WJ-2, WJ-3 and WJ-4 have not changed.

Reaching for Solvents

Greases, oils, dirt and dust, as well as drawing and cutting compounds, are a lot more difficult to remove, particularly from heavily pitted steel. Dry abrasive blasting tends to impregnate the steel with microscopic traces of non-visual contamination by pressing impurities further into the steel, creating more intensive work and further treatment to be carried out prior to acceptance if determined during inspection. For this reason, the dry abrasive blasting standards should always be used in conjunction with SSPC-SP1, Solvent Cleaning.

Solvent cleaning is a method for removing all visible oil, grease, soil, drawing and cutting compounds, and other soluble contaminants from steel surfaces.

SSPC-SP1, in my opinion, is a remarkable standard for pre-cleaning treatment prior to primary or secondary surface preparation, and if used correctly and implemented into a project, it can greatly influence the end result in terms of both visual and non-visual surface cleanliness.

The standard incorporates a number of surface cleaning methods ranging from solvent cleaning, to emulsifying, to steam-cleaning. These can be applied in a number of ways, from wiping and scrubbing, to solvent spray, to vapour degreasing.

A major problem with this standard is that not many realize that the SSPC-SP1 standard is intended for use in conjunction with the dry abrasive blasting standards for surface preparation per SSPC-VIS 1, and the requirements are more often overlooked and therefore not carried out.

How many inspectors have shown up to a site for a coating project and along comes the contractor with his gleaming blasting equipment and blast hoses, but no fresh water for wash requirements, nor steam cleaning equipment or emulsifying agents? I am sure many of you are familiar with this situation. Many believe that dry abrasive blasting alone is capable of removing contamination; however, as can clearly be seen, it is not.

Picking the Right Method

The surface preparation method required usually depends upon a number of considerations and factors. Selecting the proper method depends on the substrate, the environment, the coating selected and the expected service life of the coating system. Economics, surface contamination and the environment will also influence the selection of surface preparation methods.

There really is so much to consider when engineering a protective coating program, whether it be new-build construction or maintenance. There are many areas where dry abrasive blasting cannot be carried out or considered, either for economic purposes/costs and safety reasons, or because of factors such as the presence of delicate or sensitive machinery close to the area to be prepared. In areas like this, alternative methods have to be used.

I have only touched on some of the factors to consider with dry abrasive blasting and water jetting, showing only some of the pros and cons of both methods of surface preparation. Hand tool cleaning and power tool cleaning bring along their own advantages and disadvantages, and I have yet to discuss surface preparation of water with abrasive mix (or slurry) and shroud blasting, which are blog entries in their own right. However, the above just shows how vast the range of technology and science behind surface preparation really is.


Lee Wilson

Lee Wilson, CEng, FICorr, is a NACE Level 3-certified CIP Instructor, NACE Corrosion Specialist, NACE Protective Coating Specialist and Senior Corrosion Technologist, as well as an ICorr Level 3 Painting Inspector and Level 2 Insulation Inspector. The author of the best-selling Paint Inspector’s Field Guide, Lee was named one of JPCL Top Thinkers: The Clive Hare Honors in 2012. Contact Lee.



Tagged categories: EMEA (Europe, Middle East and Africa); Engineers; Inspection; Institute of Corrosion (ICorr); Lee Wilson, CEng, MICorr; NACE; North America; Quality Control; Quality control; SSPC; Abrasive blasting; Certifications and standards; Solvent and chemical cleaning; SSPC VIS 1; SSPC-SP 10; SSPC-SP WJ-1; SSPC-SP WJ-2; SSPC-SP WJ-3; SSPC-SP WJ-4; Surface Preparation; Surface preparation; Waterjetting

Comment from Mark Edmonds, (7/24/2017, 2:36 PM)

Hi Lee, You note in the disadvantages of water jetting that it "does not remove" mil scale." The current WJ-1 standard says the surface has to be free of mil scale. Paragraph 2.1.1 of the standard says thin films of mil scale ....are not allowed. Are you saying water jetting will not remove mil scale even though for WJ-1 it is not allowed?

Comment from Lee Wilson, (7/26/2017, 8:45 AM)

hi Mark, do a copper sulfate test on a WJ1 always trace elements of mill scale the standards wrong my friend they tried to collaborate it with the blasting standards which in my opinion didn't work

Comment from Mark Edmonds, (7/26/2017, 12:32 PM)

Lee, thanks for this information. You answered my question perfectly.

Comment from Lee Spoor, (7/27/2017, 11:42 AM)

Hi Lee, a really nice blog which outlines the different surface prep techniques and pros & cons of each. I imagine that one of the "take home" points of this is that it really is a case of "horses for courses" as there's no right or wrong method. If nothing else, this is a really good refresher for anyone involved in surface preparation, particularly with regards to the different standards. Many thanks!

Comment from Lydia Frenzel, (7/27/2017, 8:34 PM)

I recognize your blog is not for details. The WJ Task group has always maintained that Wj is for surfaces that have already been painted or has a profile. It reveals the profile under the paint or corrosion. It is not for new construction, although the Brazilians use it for new marine construction where the steel has gone through a blast cabinet. On the amount of water, equipment has been avi\ailable since 1994 to collect the water and paint, separate it, and reuse the water. The economics of the project drives the recycling or a more simpler separate out most of the solids and then go to industrial waste water streams. On mill scale, the guide photos on a multi-coat system shows black specks of mill scale that was on the substrate during original construction for WJ-2. Then for WJ-1, the mill scale is removed. The point is- there are specks of mill scale left on plates taken through a blasting assembly that are acceptable, painted over, and work. I have seen in the field areas where the mill scale is removed by WJ, and other areas where the mill scale is NOT removed.Some mill scale will pop off. I have slowly removed intact mill scale solely with UHP WJ- but it is slow and wouldn't be used in the field. Over the years, I have become very jaded with the hypocrisy of the paint inspectors wrt mill scale in the field. WJ takes the paint off and reveals mill scale- generally specks or isolated spots- which have been on the substrate since the construction. The system hasn't failed, but somehow if WJ reveals it, the substrate is no longer suitable. I'm not talking about the substrates that were painted with no original mill scale removed. I'm talking about vessels that have been in the field for years with little islands of mill scale that was perfectly acceptable at time of construction. Your picture for WJ appears to show new pipe being cleaned with WJ. Sid Taylor of INCAL has installed plants for old pipe to have the exterior and interior coatings removed by WJ, not new steel pipe. The pipe is inspected for corrosion, and might be put back into service, or might be used as conduits, or might be sent to steel mills, depending on the wall thickness, pitting, etc.

Comment from Mark Edmonds, (7/30/2017, 10:15 PM)

To Lee and Dr. Lydia, This is my take on the conversation. We have two highly respected individuals in our industry, one with more inspection initials after your name than Google has Alphabets and another undisputed expert in using waterjetting as a surface preparation method. Both of you agree that mill scale will remain after a WJ-1. The problem though is as an inspector the SSPC/NACE standard does not allow mill scale, even though according to Dr. Frenzel a small amount of mill scale is not a problem. It seems to me an effort needs to be made to come to a consensus about mill scale on WJ-1 prepared steel and concurrently maybe changing the specification to match the decision about leaving mill scale. If it is determined that all mill scale has to be removed then the WJ-1 spec should be amended accordingly. I can hear the screaming now from Pittsburgh and Houston that this is way easier said than done but it doesn't change the cloud this blog has put on the accuracy of the current WJ-1 spec.

Comment from Lee Wilson, (8/3/2017, 12:05 AM)

Mark Edmonds poetry in motion

Comment from Lee Wilson, (8/9/2017, 11:59 AM)

Lydia I have wrote an article on the WJ standards and the problems the industry faces and asked the question Waterjetting standards does the revision of NACE No5/SSPC SP 12 work? I hope it will soon be published as can be seen its quite a hot topic in the industry as present.

Comment from Lydia Frenzel, (8/14/2017, 7:00 PM)

We show, in the photographic guide, Grade C and D steel, and previously painted surfaces. In today's terms, this means that there is no mill scale on the substrate. Why do we talk abut mill scale? Because in older structures, when the paint is removed, it it not uncommon to see specks of mill scale. They were there from the beginning. So the cleanliness is WJ-2 if those specks of mill scale remain on. It becomes WJ-1 when they are removed. We have a photographic illustration which was part of Hempel's series. We do not show Grade B. I have seen mill sale popped off in field applications. I personally have taken mill scale off of Grade B steel- but it is slow, and not something that you would do in the field. It's not my personal opinion, one way or another, to say that specks of mill scale are suitable to paint over. However, specks of mill scale are found in the field so someone made that decision at the time of new build. Waterjetting uses lots of water, but it can be recycled. Abrasive blasting uses lots of abrasives, but abrasives can be recycled. Where cost saving enter is that the solids can be separated from the water fairly easily. The effluent water is disposed as industrial waste water- often in the plant water system. The amount of waste generation to go to a "hazardous" site is much less than a typical dry blast project. Cost savings come at the disposal consideration. When removing mill scale in the field, the difficulty comes in identifying it. If mill scale is still intact, it can look like a black tightly adherent primer. Nozzle operators (jetters) can lose weeks trying to hydro-mill the layer off as they think it is paint, rather than mill scale. My advice when they hit more than specks is to switch to a Wet abrasive head (if abrasives can be tolerated), or to a rotating wire brush- if the patches are small.

Comment from Lee Wilson, (8/17/2017, 12:38 PM)

Hi Lydia some excellent remarks and comments to which I agree and concur with however the original standard has been subsequently withdrawn and replaced in 2012 with what in my opinion is inferior to the original. I am led to believe that this was carried out in order to bring the waterjetting standards into some kind of correlation with the dry abrasive blasting standards. And you see this is where in my opinion it all goes wrong. These methods are 2 completely different methods of surface preparation and all that has been achieved in my opinion is a great deal of confusion. The new standards or any preceding standards should have never been linked to the dry abrasive blasting standards! Above all else these standards are confirmed by means of a visual inspection and any qualified inspector across the globe will tell you that the appearance of a dry abrasive blasted substrate compared to that of a waterjetted prepared substrate are completely different. My point is in an attempt to simplify and perhaps amalgamate 2 methods of surface preparation they have created a great deal of ambiguity and confusion within the industry. There are also concerns that the new standards have dropped the guard in regards to quality requirements. The new WJ standards state in Paragraph 1.1.1: “Clean to Bare Substrate is the waterjet cleaning equivalent to ISO 8501-1 Sa 3 Cleaning to Bare Metal.” Paragraph 1.1.2 states: “Within the hierarchy of degrees of surface cleanliness achieved by waterjet cleaning, Clean to Bare Substrate (WJ-1) is intended to be similar to the degree of surface cleanliness of NACE No. 1/ SSPC-SP 5 except that stains are permitted to remain on the surface.” In addition, Paragraph 3.2.3 states: “Direct correlation to existing dry abrasive blasting standards and visual comparators is inaccurate or inappropriate.” Malcolm McNeil, CIP Committee Chair of NACE CIP in 2013 stated within inspect This! In the summer of 2013 edition the following “Depending on how you interpret these statements, they appear to be in conflict with each other and somewhat confusing to the reader. Therefore, it is recommended that for clarification, if needed, one should contact the committee responsible for writing the new standards”.

Comment from Lee Wilson, (8/17/2017, 12:41 PM)

Now here is my point as an inspector I don’t want to contact the committee responsible for writing the new standards as I want the standards to be clear non-conflicting and non-confusing. In 2013 Lydia M Frenzel submitted a technical paper for the 2013 WJTA-IMCA Conference and Expo in which Lydia asked the question when will we ever get a standard based upon water plus abrasive injection blasting or WA blasting. In the paper Lydia set out the problems encountered in developing such a standard and stated the following: “It seemed painfully obvious in 1985 that the dry abrasive standard language could be modified slightly to include the effect of rusting when water was present. It took about 15 years and three chairmen before the chairmen accepted that this modification just would not work. The Dry Abrasive Blast Cleaning language was too specific. To this day, the industry continues to delude itself with the idea that the contractors might use water and abrasive because of dust or salt issues, but the users want the production rates, the width of the cleaning pattern, and the appearance to be exactly the same as dry abrasive blast cleaning”. I have to concur with Lidya. The attempt to correlate the water jetting and abrasive blasting although an ideal if it could be achieved is simply a fantasy and attempts in order to achieve this fantasy are resulting in widespread confusion.

Comment from Lee Wilson, (8/17/2017, 1:14 PM)

). One of the most formidable changes within the new standards is the removal of the non-visible contaminants. As per the original standard there were 3 designations for the removal of non-visual contaminants which ultimately relate to the removal of soluble salts, chlorides, sulphates and nitrates from the substrate these where designated as NV-1, NV-2 and NV-3 and each designated the amount of non-visual contaminants that was permitted to remain on the substrate after preparation. This is a major benefit of actually using waterjetting as a method of surface preparation and was of additional use to engineers in coating selection and of course inspectors and contractors carrying out the works as guidelines where clear upon the NV restrictions and min / max allowances. Unfortunately, these requirements are no longer applicable to the standard with the responsibility for NV acceptability now being in the hands of specification writers. I have to ask why? Taking into consideration the huge failures contributed to excessive salt levels remaining upon the substrate prior to coating application and the subsequent osmotic blistering, lifting and disbondment attributed to these contaminants has this been left to the specifier? I also have to also ask what is a standard? It is of my opinion that a standard is a document that provides the requirements, specifications, guidelines or characteristics which can be used consistently to ensure that materials, products, processes and services are fit for purpose. I have to also ask is this standard now fit for purpose? I ask this question as the main purposes of this method of surface preparation is the removal of non-visual contaminants and the acceptable non-visual acceptance and rejection criteria’s stated within the standard. With these now left to the specifier I do believe that this key element is now lost and this is causing the concerns within the industry in regards to potential quality issues. Simply if a standard cannot set out the requirements for acceptance and rejection criteria then we have to ask what use is the standard?

Comment from Harry Peters, (9/27/2017, 11:52 AM)

Lee: It has been unfortunately presumed that UHP WJ removes soluble salts to levels which do not require further concern with respect to these non-visible contaminants. In our experience this has not been the case, especially with heavily weathered steel or steel in service for extended periods in marine environments, and in large part due to use of unsatisfactory testing methods. The use of UHP water jetting offshore (see the case study in CoatingsProMag, March 2017, pages 46 to 52 as an example), in produced water tanks in thermal oil recovery operations (see the MP article "Eliminating Coating Failures..." ,f October 2014, pages 38 to 42) and a project internally recorded on the USS Saipan ( website under case studies), all demonstrating that different services in which UHP waterjetting has been insufficient to desorb surface adsorbed salts. Lastly, an article by Neil Wild "Surface Tolerant Coatings for Offshore Maintenance" duplicates the results of an extensive EU study on surface preparation that UHP water jetting only lowered levels of chloride contamination to around 20 micrograms/sq. cm. As such, it is fortuitous that the NV limits in the original water jet standard were removed in the presently used revision, both because commonly used equivalency testing could not easily verify such limits in the field, and because the NV limits were being misused in other non water jetting specifications. The issue of residual surface salts continue to plague the industry with premature coating failures and it is an issue which the industry and its associations are addressing. SSPC has Guide15 which outlines various commercially available testing methods, though the predecessor TU4 SSPC document had more robust and cautionary information to assist the potential user. NACE published 6G186, which addresses the awareness and concerns associated with residual surface soluble salts. Further, NACE recently published a consensus standard, SP0716, which outlines the frequency and locations of tests for residual surface salts during surface preparation prior to the application of coating. What follows is the committee work of TG546 which is developing a proposed NACE standard: "Soluble Salt Limits Based on Service Environment". The industry is more prepared than ever to address this issue because the scientifically factual data on the issue is unwavering in addressing the direct correlation between coating performance and the presence of residual surface salts. Also, more owners, aware and being impacted financially by premature coating failures have become more involved to seek consensus limits, best practices for salt testing and mitigation, and seeking extended warranties from coating manufacturers. In turn, coating manufacturers are more engaged in order to meet the demands of their owner-customers. On completion, the WJ standards will be able to refer to the NACE cleanliness salt limits which is more appropriate since the WJ standards generate results for visual appearance, mirror imaging the dry blast standards. The newer committee generated standards and proposed drafts address non-visible concerns.

Comment from Lee Wilson, (9/27/2017, 1:59 PM)

Hi Harry some valid points and some good comments I will get to the NV situation later as that an article and a debate within itself however can you explain how the WJ standards generate results for visual appearance which mirror image the dry blast standards because I can't for the sake of me see how this can be achieved from a visual perspective! Nor can the industry as per all comments above!

Comment from Harry Peters, (9/27/2017, 5:14 PM)

What I am suggesting is that there are relatively simple visual standards for dry blasting, with pictures to verify, along with relatively simple visual standards for water jetting, with pictures to verify, and that these do not deal with non-visible standard requirements for complete surface preparation.

Comment from Francisco Giaquinto, (4/24/2018, 4:48 PM)

Congratulations Lee Wilson and Lydia Franzel for the technical comments. They could only guide these comments, technicians who attended works of this execution and are able to confront Normas. Here in Brazil I have a lot of conflicts with the inspectors of industrial painting (qualified technicians). They are based entirely on the Standard and do not accept its modifications (evolutions). I see discrepancies in treatment and application that run away from "common sense". So thank you both for such an initiative. They are true trainers of professional opinions. What I could add, with my thirty of the works in industrial painting in all the plants I worked on, is the treatment solution where there is no electrical equipment that can not be isolated and / or another factor of exposure to water, which is Blasting Wet Abrasive. And I go further, the ideal to have watched when it was allowed, would be this jet with sand. I always commented that the sand extracted from the river, which is sandblasted with water in high pressure, after the use of easy separation of the paint and inpurezas, can return nature without any aggression to the environment, which other abrasives provide. There has been prohibition in Brazil since 2004 of the use of dry or wet sand, in the prevention of silicosis by the spray of the silica powder Law very poorly worded since the wet jet is incorporated into the vehicle water under pressure and does not produce anywhere in the process the powder. With the moist jet the impurities are reduced significantly. thankful; Francis

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