Dunno much about the softening primer, but if you are using Rustoleum and you still have the "old" paint on your car you don't need to use primer except over the repair spots, may even be easier to paint over the repairs with Rustoleum rattle can of similar color. I have used Duplicolor primer and another brand od self etching primer and did not have good luck with paint sticking to it.

Could be you need to let the primer cure for a week or so fisrt.

If painting over old paint, just rough it up with about 400-600 sandpaper. And roll away.