Surface Preparation Standards in Paints and Coatings
Proper surface preparation is essential for the success of any protective coating process. The importance of removing oil, grease, prior coatings, surface defects and contaminants cannot be overemphasized.
The performance of any paint coating is directly dependent upon the correct and thorough preparation of the surface prior to coating. The most expensive and technologically advanced coating system will fail if the surface preparation is inadequate, incorrect, or incomplete. The main objective of surface preparation is to ensure that all such contamination is removed, imperfections are removed or minimized, and surface tensions or profiles are established as recommended.
Surface preparation is dependent upon many variables. Whether metal or composite, there are dozens and dozens of substrate types utilized in today’s manufacturing of painted parts and components. There are many types of metals, and many types of plastics and composites, all with unique requirements for achieving optimal adhesion and performance.
Cleaning new, raw plastic parts can be simple or complex. A lot depends on the quality of the plastic, virgin or regrind, mold-release or no, the quality of the tooling and the forming process, and the handling of the part from forming all the way until it is delivered to the painting process.
Some plastics are paintable, or paint-grade, but in general, plastics are made so that things not to stick to them. Depending on your plastic, sometimes a simple clean air blow-off is sufficient. Or maybe a soap and water wash might be sufficient. In many environments, it makes sense to utilize a multi-stage washer with chemical wash and DI water rinse. pH, temperature, pressure, and volume of the wash and rinse will play a vital role as well. The wash is designed to remove any contaminants including mold-release agents left behind from the molding process. Maintenance of the wash and rinse water is vital to the process’ success.
Sometimes an isopropyl alcohol wipe of the parts is utilized to remove similar contaminants, in addition to, or in place of the water wash. Depending on the environment, or if parts are being reworked, sometimes a wax and grease is used.
Depending on the plastic, sometimes more aggressive surface prep is required. All are designed to help modify the surface tension of the plastic to better receive the coating. The bonding can happen chemically, mechanically, or both.
- Sanding or scuffing
- Scuff pads vary widely in coarseness, and all depends on your application.
- Flame or plasma-treat
- Chemical or solvent prep
- Blast cleaning (steel, sand, or shot is typical in steel prep, but CO2 under pressure is somewhat common with plastics)
Anti-Static and Adhesion Promoter
Because plastic easily takes on a static charge and plastic parts with a static charge attract dust, an anti-static process is recommended. This is typically performed with an ion-charged air blow-off. There are also some commercial wipers available that are designed to eliminate the static buildup on the surface of parts.
Plastics that are not paint-grade, typically require an adhesion promoter. “Adpro” is designed to chemically bond with the plastic and create a “canvas” for the next layers in the process.
Some paint-grade plastics do not require adpro to gain adhesion but may contain defects that can be seen through “telegraphing” in the paint finish. These are generally mold and splay marks created during the molding process. These defects in the plastic might be a raised area where the tools join, or might be injection points, or might simply be created from varying temperature in the mold. In those “splay” areas, the surface tension of the plastic will vary, and can cause orientation issues with the paint. A sealer coat is required in this case. The sealer will act as a cover, or primer, to bury these spots and create a uniform canvas with consistent surface tension for the base layer of paint.
If mold defects are too great, resulting in peaks or valleys in the part, sometimes a primer/surfacer is required. These are common with metal applications, but sometimes are required with plastic (like fiberglass). In this case, a primer that builds film quickly, typically dries quickly, so that it can be sanded easily thereby creating a nice smooth finish for the paint film.
Now let’s talk about metal surfaces. Adequate surface preparation is a vital prerequisite for ensuring the quality and longevity of metal coatings. Without proper preparation, even the most advanced metal coating technologies will fail.
It is essential to remove all soluble salts, oil, grease, drilling and cutting compounds and other surface contaminants prior to further surface preparation or painting of steel. Perhaps the most common method is by solvent washing, followed by wiping dry with clean lint-free wipes or sufficient blow-off. The wiping clean is critical, because if this is not carried out thoroughly the result of solvent washing will simply spread the contamination over a wider area. Proprietary emulsions, degreasing compounds and steam cleaning are also commonly used.
Hand tool cleaning
Loosely adhering millscale, rust and old paint coatings may be removed from steel by hand wire brushing, sanding, scraping and chipping. However, these methods are incomplete, and always leave a layer of tightly adhering rust on the steel surface.
Power tool cleaning
Generally, more effective and less laborious than hand tool cleaning for the removal of loosely adhering millscale, paint and rust. However, power tool cleaning will not remove tightly adhering rust and millscale. Power wire brushes, impact tools such as needle guns, grinders and sanders are all commonly used. Care should be taken, particularly with power wire brushes, not to polish the metal surface as this will reduce the success of subsequent paint coating.
This is generally a more effective method than power tool cleaning. Cleaning is carried out by oxyacetylene torch, hot air guns, etc. This method is used to remove old paint and varnishes. While cleaning, care shall be taken not to damage or burn the substrate.
By far the most effective method for removal of millscale, rust and old coatings is using abrasives such as sand, grit, shot or CO2 under pressure. The grade of blasting suitable for a particular coating specification depends on a number of factors, the most important of which is the type of coating system selected. U.S. Paint can help you with this determination.
Prior to blasting, steelwork should be degreased and all weld spatter removed. If salts, grease or oil is present on the surface it will appear to be removed by the blasting process, but this is not the case. Although not visible, the contamination will still be present as a thin layer, and will affect the adhesion of the next coatings.
Weld seams, metal slivers and sharp edges revealed by the blasting process should be ground down, as paint coatings tend to run away from sharp edges, resulting in thin coatings and reduced protection. Weld spatter is almost impossible to coat evenly, in addition to often being loosely adherent, and it is a common cause of premature coating failure.
The surface profile obtained during blasting is important, and will depend on the abrasive used, the air pressure and the technique of blasting. Too low a profile may not provide a sufficient key for coating, while too high a profile may result in uneven coverage of high, sharp peaks possibly leading to premature coating failure, particularly for thin film coatings such as blast primers.
Wet abrasive blasting
Wet abrasive blasting uses a slurry of water and abrasive rather than just a dry abrasive. With this method you cut the hazards of dust and associated health problems. Another important advantage is that when wet blasting old, rusty surfaces, many of the soluble corrosion products in the pits of the steel will be washed out, which will greatly improve the performance of the applied coating system.
One disadvantage of the technique is that the cleaned steel begins to rust rapidly after blasting. For that reason OEMs commonly include proprietary inhibitors in the blast water which will prevent this rusting for long enough to allow painting to be carried out. Low levels of inhibitors won’t affect the performance of subsequent paint coatings for non‐immersed steelwork. The use of a moisture tolerant primer, which can be applied to wet blasted steel while it is still damp, can make the use of inhibitors unnecessary, but U.S. Paint should be consulted for specific advice. Where wet blasted surfaces have been allowed to corrode, they should be mechanically cleaned or preferably sweep blasted, to remove the corrosion prior to painting.
Hydroblasting is a technique for cleaning surfaces, which relies entirely on the energy of water striking a surface to achieve its cleaning effect. Abrasives are NOT used in hydroblasting systems. Any problems caused by dust pollution and by the disposal of spent abrasives are eliminated. Two different hydroblasting operating pressures are commonly used.
- High pressure hydroblasting, operating at pressures between 680 bar (10,000 p.s.i.) and 1,700 bar (25,000 p.s.i.).
- Ultra high pressure hydroblasting, operating at pressures above 1700 bar (25,000 p.s.i.).
Prime the surface
Priming is a very important step in preparing metal for paint, especially if the surface will be exposed to moisture. To select the right primer, the type of metal to be coated along with the desired appearance, performance requirements, and environmental conditions should be considered.
U.S. Paint is ready to work with automotive paint suppliers and other original equipment manufacturers to ensure that their surfaces are prepared for our original coatings. Let’s discuss your manufacturing process and give you the highest return on your investment. Call 314-621-0525 today.