I've used POR 15 in most of my rust repairs( surface rust
) Deep rust really needs to be removed and a patch panel made. For all my surface rust, I just did as Ted says. Take it down to bare metal with a wire wheel. Hit it with the POR 15 a couple times. Then primer and fill as needed prior to paint. I needed to fill a couple little pits that had formed from leaking acid.
I have not had any issues as of yet with the paint shrinking or popping. If you prepare the surface correctly, all the POR and filler should be sealed in with a final glaze to prevent any further absorbtion of paint materials.
And yeah POR stains everything. Takes a good month to wear off the skin.
Dave