Yeah if you’re using blue foam you have to make sure you seal it very well or use paints that won’t inst-melt it. I’ve seen some use Gesso to seal it up, then prime that a few times, then paint, not really worth the trouble. If you’re using a more dense foam then yeah, auto spray primer is your best bet. You can usually get them in rust color, black, and grey. I would usually do 2 coats, let it dry fully, then smooth it out with 400-500 grit sandpaper and some water, repeat as needed. You can actually use this primer to fill some of the more porus foams, but it takes quite a bit of time. It’s excellent on the real dense stuff. But now matter what you do, ALWAYS use the same brand of paint as the primer you choose. Once your primer is finished and totally dry wipe down the surface with soft cloth to remove hand oils, this can botch up your paint finish big time.