Clear E-Coat Over Chrome?

Q. Can a clear e-coat be applied over a chrome-plated surface (brass or zinc)? I’ve seen chrome plate listed on some websites as a surface that can be e-coated, but the sites offers no specifics. What type of surface preparation would be needed? What level of adhesion is achievable?—R.C.

A. Chrome-plated surfaces are smooth, hard and very chemically inert so that, although a clear electrocoat could be deposited over it with reasonably good initial adhesion, it would be very difficult to obtain adequate adhesion and durability over time. Some kind of adhesion promoter (must be conductive) or light acid etch (to provide a rougher surface) would be necessary as a pretreatment to enable the clear e-coat to deposit over the chrome-plated surface and provide improved adhesion and durability.

There are companies that claim special pretreatments provide the long-term adhesion and durability necessary to e-coat over chrome-plated surfaces, but I can’t confirm this. I have never known anyone to do it. Care must be taken to ensure that the special pretreatment does not introduce any color to the substrate and therefore cancel the benefit of the clear e-coat over the chrome plate.

Typical automotive zinc phosphates do not improve adhesion over chrome-plated surfaces, as there is no iron in the substrate for the phosphate to properly develop.