Paint Compatibility

Q. I run a job-shop painting operation that also does remedial painting on a contract basis. One of our customers has 48 steel valves ranging from from 6–48 inches. These valves were supplied to our customer coated with a high temperature one-pack silicone coating system. We have been instructed to repaint these valves using the following paint system:

1)Inorganic zinc silicate primer,
2)High build epoxy/polyamide primer,
3)Acrylic/polyurethane red gloss enamel.

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Because they are in service, we cannot blast these valves; power tool cleaning is the only form of surface preparation. Therefore all the existing coating cannot be removed. Is the proposed system compatible with the existing silicone-based paint? Q.B.

A. The simple answer is no. You actually have two problems. The first is adhesion of any coating to silicone-based paint. It’s well known that most paints won’t adhere to silicone coatings. The second is that inorganic zinc silicate primers must be applied to bare metal where they react and form an adhesive bond with the substrate. To solve both problems, the silicone-based paint must be completely removed.

The valves being in service further complicates your problems, eliminating the option of blast cleaning. Use of chemical strippers could be an option, depending on the valves’ locations. I’d consider a combination of chemical stripping and power tool abrasion to completely remove the existing silicone paint before repainting.