It can be difficult to paint new galvanized steel in the field. Because galvanized steel is pre-passivated to prevent humidity and storage staining, most paint systems do not adhere well to it. While advancements in paint and technology have occurred in the last decade, prepping galvanized steel for painting should be handled with care.
If possible, it’s recommended you purchase a galvanized product designed for painting like Galvanneal or phosphatized/bonderized (phos-bond). Both of these products are specially produced to be painted and are ready for painting with no prep.
However, let’s assume you have your hands on a standard G90 sheet. You can prep it for painting with these five steps:
Weather it or blast it
When a sheet is left to “weather” for at least six months, Galvinfo tells us that the chromate film will have washed away and the surface will have begun to oxidize. The light oxidation allows the paint to adhere more readily, so weathering is a good way to prep the steel. Once weathered, all that’s needed is a warm water wash to remove any accumulated debris.
If time is an issue, you can also attempt to “blast” galvanized steel. Be aware that this technique isn’t without its drawbacks. Along with requiring high levels of expertise and intense monitoring of particle size, angularity, freshness and cleanliness, it can be cost-prohibitively expensive.
Environmental contaminants like dust must also be taken into consideration, and over-blasting can remove too much zinc, which exposes the base steel, leaving it vulnerable to corrosion. On the other hand, under-blasting won’t remove enough of the chromate film to be effective.
Ensure its water break-free
Regardless of the method you choose, the next and most critical step is to check if the material is water break-free. Galvinfo says water break-free means there’s no beading of water when the surface is wet.
“If it is not, then it must weather for a longer time, or be treated with a phosphoric acid or zinc phosphate before painting,” they add.
Squeaky clean and dry
After confirming that the material is ready for painting, it’s important to remove all dirt, rust, oil, grease and debris. Though you should use soap and water for this process, ensure that any traces of soap are fully removed, and paint only when the surface is completely dry.
Consider a solution
Galvinfo notes that numerous proprietary pretreatment solutions are available from many suppliers.
They recommend using one – like a zinc phosphate – for adhesion and to extend the service life of the paint.
Consult your paint supplier
From here, it will depend on the paint system you choose and the supplier’s usage instructions. For some applications, you’ll want to use a primer that’s compatible with your topcoat. And keep in mind that many paints designed for wood won’t perform well on galv.
According to Galvinfo, the most important thing in this stage is to find a paint designed specifically for metal surfaces. If you don’t, the paint could react with the surface of the steel, which may cause the paint to peel.
Opt out of it all
As we mentioned in the intro, some companies including Madison Chemical Industries have been hard at work manufacturing modified paints and products that eliminate the need for blasting or weathering.
These high-performance polymer coatings don’t require the typical surface prep, and promise to keep the zinc layer intact. They also promote optimum adhesion, which saves money, time and manpower.
