October 19, — Case study
4 tips to optimal surface preparation
The life of a coating system depends as much on the degree and quality of surface preparation as on the selected coating system.
It is a lot harder and tedious to prepare a surface than to coat it. It will generally take more time for the preparation than the coating application itself. A lot of us will tend to skip or go quick on this important process. Yes, it’s true, preparation does not necessarily show after a part is coated, but even if you use the most expensive coatings, a contaminated surface or badly prepared surface will likely lead to failure or premature delamination. If you have contaminants on the surface, eliminate any future risks with an adequate surface preparation. If you want to compete with the best players in the industry, invest the time to prep your surfaces right. You will be amazed at the results !!!
3 CONDITIONS FOR AN EXCELLENT SURFACE PREPARATION
1. The surface is adherent and has the right surface profile
2. Ambient temperature is above 10⁰C (50⁰F)
3. The surface is free of the following contaminants :
- Oil & grease
- Dirt & dust
- Rust & corrosion (for metals only)
- Combustion residue – carbon
- Mill scale (for steel and stainless steel only)
- Soluble salts
- Deteriorated paint
- Alkaline degreasers – water-borne
- Greensolv Solution: G-Max 328
- Emulsions – a combination of solvents and alkaline degreasers
- Greensolv Solution: G-Control 812
- Solvents – ex: acetone, mineral spirits, toluene, etc.
- Greensolv Solution : Greensolv 935 & 940
Using Alkaline degreasers OR an Emulsion is highly recommended. Stay away from solvents as much as possible. Our own Salt spray tests have shown that Alkaline degreasers offer up to 5 times more coating longevity (salt-spray hours, ASTM B117) versus solvents.
Water-borne alkaline degreasers and emulsions however need to be rinsed off with water. This will inevitably cause flash rust on carbon steel surface. It is recommended to use a corrosion inhibitor in the rinsing water to avoid flash rusting. But most corrosion inhibitors will harm paint adherence because they are oil-based.
Prevent flash rust: Use an amine corrosion inhibitor which will not harm paint adherence on steel. Greensolv Solution: Use VPCl 440 at 0.25% in water.
TIP #3 CLEAN THE SURFACE AFTER A BLAST CLEANING
Blast cleaning is most commonly performed with one of the following: sand (for aluminum and steel), aluminum oxide (for aluminum and steel) and steel grit (for steel only). Other media can also be used.
When recirculated in a closed system, the abrasives get contaminated with oil, grease, carbon residue, mill scale, corrosion and so on; which can be redeposited on the blasted surface, thus contaminating the surface once again.
Greensolv Solution: G-Max 328 with a corrosion inhibitor (VPCI 440) in the rinsing water.
TIP #4 USE AN ACID WASH TO REMOVE SOLUBLE SALTS AND LIGHT CORROSION
To remove light corrosion and soluble salts (this problem occurs more often in salted water environment), use an acid wash. The most popular acid wash is phosphoric acid based; it converts the light adherent rust into iron phosphate (black color). Note that phosphoric acid is now highly regulated in waste water streams because it is a bio-accumulator (life promoter) that can cause blue agae in lake and waterways. Other acid wash can be available on the market. Beware of flash rusting on steel surfaces when rinsing with clean water.
Greensolv Solution: G-Clean 257
Salt Spray test
The most recognized test to determine the longevity of a coating in the industry is the Salt Spray test (as per ASTM B117). In short, you prepare and coat a small metal coupon with the same surface preparation and coating system as you would use in your regular production. With a sharp knife, you cut the coating in the center of the coupon all the way down to the metal. This coupon is then placed in a cabinet saturated with humidity and salt. Regular inspections and measurements of the delamination around the cut are performed until it reaches a certain point. The results are measured in hours. A good result is above 1,500 hours – 62.5 days (this is from my experience, but large corporations and governments could even ask for 2,000 hours and more). Normally, the customer decides what quality he is looking for, so the number of hours will vary greatly.
By : Martin Pageau, P.Eng.
Martin Pageau is a specialist in metal treatments. His chemical and engineering background and experience in the field makes him one of the most renowned in this field.