Sealant Adhesion to Stainless Steel

QUESTION:

We have a line of products for commercial buildings. One of our problem areas is bonding sealants to stainless steel. Is there any particular alloy or surface treatment of stainless steel that would improve the adhesion of weatherproofing sealants? Some silicones bond pretty well, but urethane sealants generally require a primer (actually a silane adhesion promoter). Over the years we have had mixed results with adhesion testing, but have never properly identified the particulars of the stainless samples. M.S.

ANSWER:

Stainless steel is relatively inert, hence adhesion of paints and sealants is often a problem. The first step is to remove any oily soils. Adhesion to stainless steel is enhanced by using pretreatments. Acid etching is a pretreatment. Surface abrasion is a pretreatment. Primers are also pretreatments. Although it is effective, I don't like to recommend hydrofluoric acid etches because of the extreme safety hazard. It must always be used under controlled conditions. Instead, muriatic acid may be just as effective. Another possible problem with using acids on certain grades of stainless is pitting and stress corrosion cracking. You can eliminate the acid and roughen the surface by abrading. This can be done by using sandpaper, plastic abrasive pads, wire wheels, wire brushes, or media blasting. Again, it is important to note that the first step in any pretreatment is surface degreasing. After degreasing and abrading, apply the sealant as soon as possible.