Painting Stainless Steel

Q. I manage the finishing operations at my plant, where we produce equipment cabinets using cold-rolled steel. We are developing a special-order cabinet for one of our customers using mostly 41003 (a utility-grade stainless steel that is 12 percent Cr) and either 304 or 201 for the hard-to-draw parts. The unit has been prototyped, but we are concerned about paint adhesion. We have what is considered a high-quality paint line consisting of phosphate pretreatment, e-coat and topcoat. The e-coat people tell us that the adhesion is contingent on the phosphate, and the phosphate people tell us that there will be no phosphate coating on 304 or 201, and minimal or sporadic coating on the 41003 material. We are getting little or no phosphate coating on the stainless, and our e-coat primer, even on grit-blasted samples, seems to have poor adherence.
I am struggling to find anyone who has successfully done high-quality painting on any type of stainless steel. We would like a continuous process similar to what we have, but are afraid we may need to abrasive blast and switch to a wet “direct-to-metal” primer, which will likely preclude our usual high-quality finish. The domestic mills and paint suppliers have provided little guidance so far, and the U.S. industries have published little on painting stainless steel. Any assistance or guidance would be greatly appreciated. A.M.

A. I’m not sure why you feel you can’t get a high-quality finish using a “direct-to-metal” primer. Corrosion resistance is not a problem, because the substrate is naturally corrosion resistant. There should be no problem with appearance, provided the primer is properly applied. You have two choices for pretreating stainless steel prior to painting: abrasion or wash primers.

Q. That is the problem—we will need to find a wash primer that will work with a primer that is compatible with our topcoat. We also need it to be able to handle a fairly high paint bake, due to the adhesives and topcoat requirements. Our topcoats are pretty thin and designed to go on e-coat. With direct-to-metal, we may have drip, sag and other issues our topcoat will highlight. If we need to go to a new topcoat system, there would be significant investment. There is also concern that direct-to-metal will only be viable for visible locations. If welded areas that can trap water are not coated, those welds could be compromised. A.M.

A. Blast abrasion may be your best bet. The profile doesn’t have to be too high. Just a light blast should be sufficient. Finding a primer that is compatible with your topcoat should not be a problem. Just ask you topcoat supplier to recommend one.