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Hi Bob, It can be done either way. Many, perhaps most, prefer bare metal only, but it's not absolutely necessary. It's sort of like the epoxy vs. etch debate. I put Evercoat "Chrome-A-Lite" filler over the MP170 epoxy regularly without problems. I prefer it that way because that fits my situation and workflow better than the other way around. I would ONLY do that with an epoxy as the primer though. In fact the fillers that PPG markets themselves specifically indicate their epoxy primer (DPLF) is an acceptable substrate for their filler. The PPG website doesn't appear to be functioning properly right now or I would link to their filler tech sheet. Just keep your fills as thin as possible - try for nothing greater than 1/8" thick, no very large areas and scuff up the MP170 well with 80 grit or coarser. You won't have a problem. Here's what Evercoat has to say about it in their FAQs: "Q. Can Evercoat fillers be applied over bare metal? Can they be applied over paint? A. Our fillers are designed to work over bare, properly prepared substrates such as: steel, aluminum, galvanized, stainless steel, fiberglass, and SMC. Some people prefer applying an epoxy primer over bare substrates to enhance corrosion protection. Our products don’t need to be applied over an epoxy for corrosion protection as long as the bare surface area is clean and no surface rust or contamination is present. However, some auto manufacturers do require body technicians to coat the bare metal surface with an epoxy before applying fillers. If you are performing warranty work, you should consult the manufacturer of the automobile for the recommended procedure. Fillers and putties will normally work OK over properly sanded (80-180 grit) cured OEM paint. However, with so many different types of aftermarket paint available (lacquer, enamel, urethane, water-based). We recommend that all paint be removed where filler is to be applied." Source url for above quote. Rod
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