Just started working on a pump with medium to heavy pitting on the skins and on the cast pieces, have used putty filler before for the bad spots, but have'nt run into pitting this bad before. --Is there a good fill primer out there to use, instead of going over the whole skin with bondo filler?
If you go to heavy with high build primers, you can get some shrinkage in the future. I would probably use a catylized glaze then high build over the top of the glaze.
You could try a product like Feather Fill, which is more of a spray poly. It probably will not shrink like a true surfacer/high build primer. Just make sure the bare cast is epoxy primed first as the Feather Fill is not designed to go over bare metal.