[Rimrock]

Etching primer is the optimum on plastics and aluminium, brass, I have never had problems on plastics. Is not that strong concentration of acid in it, just enough for it to bite a bit into the surfaces (but keep it away from windows and other surfaces that are not supposed to be painted, as it can leave some traces when removed)
If one are going to use metallic top coat, one should seal off the etch primer with a standard primer first. The acid can react with the metal in the paint, corrosion.