Painting over bondo, paint is soaking through the primer?

[Sarah Grissom]

I have several layers of primer, wetsanded it, then rolled 3 coats of thinned rustoleum oil enamel over it. The bondo shows up as a cloud shaped distortion. I then sprayed 1 coat of rustoleum out of a can to see what would happen, the cloud disappeared but got lots of pinholes.

Do I need to use a sealer? Or do I just need to lay on many more coats?
craig, I'm doing this:

http://www.rickwrench.com/index79master.htm?http://www.rickwrench.com/50dollarpaint.html

This works really well actually, I did a couple of cars and they look terrific. On my car the previous owner sprayed high quality paint ten years ago and the pinholes were in that paint, and have reappeared when I tried sanding and repainting.