https://www.jalopyjournal.com/forum/...c838fe5adca540 Thank you for that info In 1988 I was testing some primers because I wanted the most powerful primer there was. I painted a grille shell of a 38 Packard just to try the paint out. I noticed where the metal was pitted on the second coat the pitting was disappearing. Back then a lot of us used lacquer primers and glazing putty like Nitro stain. I found this to be pretty exciting , my wife didn't. The reason you put another primer over zinc phosphate epoxy is because of the zinc in the coating causes pin holes. This is the explanation I got from the lab at CFI. Slick Sand & Feather fill a been around for years I never needed it. I introduced this product to the hobby under the banner Get Rid of the Pits .I am retired now ,not doing too much painting It's been 32 years I still have that piece.