Yes, spray paint will melt some of the foam, but not as much as you think. I used the paper coated board from a Dollar Tree and painted it after I put a coat of resin and Bondo-Glass. Some of the foam did melt, but only about 1/4 inch in. It's easy to fill with some glazing putty.
Thin coat of plaster... Used this on a project for Bio in high school... or for this application, possibly bondo. Or fiberglas -- though the resin heats up, and could do nasty things to the foam still. Anyone else?
As the originiator of the thread here is my update.
The project that I was going to do this for was changed and am no longer using foamcore. Something better presented itself.
In the past I have coated regular styrofoam with a couple of coats of acrylic gesso and the spraypaint was fine.
I have seen spray paint in the fabric stores that is specifically for use on styrofoam.
I've never used it but I knew folks who did. Keep in mind this was 12 years ago so I don't know if this stuff is still around.
I read somewhere that the Krylon H2O rattlecans are styro safe but I have yet to see any proof of that.
In a nutshell, I was too lazy to rip off a chunk of scrap foamcore and spray it down. Figured I'd ask here instead.