by Ron Joseph
Painting Over Zinc Plating
Q. I have read several articles about duplex systems (paint over galvanized
steel). I am looking for information about what types of paints are compatible
with zinc plating (I assume painting over zinc plating is similar to painting
over galvanized zinc)?
A. Generally you can use the same paints on zinc plating as you do when you
paint over galvanizing. I strongly suggest that you first pretreat the surface
with either an iron or zinc phosphate process. Alternatively, you can apply
a vinyl butyral wash primer, but the phosphating process is generally considered
to be preferable to the wash primer approach. Moreover, the VOC content of wash
primers is high (often >650 g/L) and many state agencies have outlawed this
type of product.
Alkyd and alkyd modified coatings should not be used in direct contact with
the zinc as they tend to saponify upon contact with zinc in humid environments.
On the other hand, an epoxy primer followed by any of the following topcoat;
alkyd, acrylic, epoxy or polyurethane, should perform well.
Select the appropriate coating system based on the type of exposure to which
the painted product will be exposed.