My can of plastic filler says it can be applied over OEM paint. If you can keep from breaking into the metal you can skip the need for DTM primer or if the filler covers all of the bare steel, same deal. One drawback to doing it this way is that you have nothing to fill with the primer because your plastic is level with the topcoat. I do it all of the time. Sometimes I’ll fill a deep scratch with filler to keep the repair small instead of feathering everything out and filling with epoxies and polyesters.
