Adhesion to Zinc

Q. I am the assistant project manager at my plant. I have a part that is made from hot-rolled, pickled-in-oil steel plate and then zinc plated. The final step is to topcoat with high-temperature-resistant paint, but I cannot get the topcoat to adhere to the zinc plating. We tried washing the part with an iron phosphate wash, and it helped some, but we still did not get 100-percent adhesion. Can anything be done to the part to get the topcoat to adhere? B.H.

A. Zinc surfaces are very active. Without getting into the exact chemistry, certain resins saponify (form a soap) when coated on zinc, causing the paint film to lose adhesion. To get paint to adhere to zinc surfaces, you must either passivate the surface or change the paint. Zinc phosphate pretreatments can be used to passivate zinc surfaces such as zinc-plated, galvanized or galvanneal substrates prior to painting. The other alternative is to change the paint to one that is not affected by zinc. Oil-modified paints are extremely susceptible to saponification over zinc surfaces, while epoxies, for example, are not.