I am in the process of repairing a rust spot on my car. I sanded all the rust down. I put body filler on it and used glazing putty. I couldn't get that stuff to come out good enough so I used a filler primer. The filler primer came out really well, especially after I sanded it down. I am painting it white so after that I put on two coats of white primer (I am painting it white) so that it would get covered by the actual paint easier. The white primer, however, came out with hundreds of pinholes and voids after I sanded it (600 grit). Any advice?

Hmmm. Sounds like it could be solvent pop but need some pics first, please.

Please post pictures. Too difficult to guess at what you are actually seeing.