Depends on how deep your pits are? You could try a sandable primer and put a few coats on and sand or try a high build primer. Did you blast prior primer? Or wire brush? Maybe sand your filler better? You could try skim coating with a product called Icing from NAPA. I don't know how it works with motor temperatures or heat but is great on bodywork. Another product called Lab Metal may work. Hard to tell without seeing. Maybe posting a picture would help. Good luck and have fun
Dan
