The usual pre-treatment is conversion coating, either chromating or phosphating. Phosphating is probably more commonly used although chromating gives better corrosion resistance. There are many variations; the easiest (which avoids setting up a processing line or out-sourcing the work) is to use a ‘wash primer.’ Also called an etch primer or acid-resin coat. This is a two part mixture: one consists of a polyvinyl butyl resin dissolved in alcohol (+filler/pigment), the second is a phosphoric acid-water solution. Mix just before application. When applied to the zinc or zinc alloy, the phosphoric acid reacts to form a very thin phosphate. As the alcohol evaporates, it leaves a very thin resin coating which protects the phosphate and improves paint adhesion. It is important to only use a thin application so that there is no unreacted phosphoric acid left in the film.
For resin-acid wash primers incorporating zinc chromate pigment, see DOD-P-15328 and MIL-C-8514. Both are available at the DoD’s ASSIST site:
Also ‘Paint No. 27 Basic Zinc Chromate-Vinyl Butyrol Wash Primer’ -- Steel Structures Painting Council Specification.
See also
ASTM D2092-95(2001)e1 Standard Guide for Preparation of Zinc-Coated (Galvanized) Steel Surfaces for Painting.
MIL-C-17711 Coatings, Chromate, for Zinc Alloy Castings and Hot Dip Galvanized Surface.
MIL-T-12879A Treatments, Chemical, Prepaint and Corrosion Inhibitive, for Zinc Surfaces.
Metal Finishing Guidebook & Directory and
Metal Finishing Organic Finishing Guidebook & Directory. Both are included with a subscription to
Metal Finishing magazine.
As for painting, it depends upon the parts’ application and available painting equipment (wet spray, powder coat, ovens, etc.). An epoxy base coat and polyurethane topcoat is common for outdoor applications. I have also heard that it is possible to (anodically) ecoat diecast alloys without pretreatment, but I am not familiar with the process.
