Goolies, which foam are you working with? Mac and I are referring to HDPU (High Denesity Polyurethane), if that is what you are using I have seen the pin hole that becomes visible after the first primer coat. If that is your problem try a thinned out dip into Zinsser Bulls Eye 123. A bright white shellac product that dries quick and seals well. Just decant some into a pint or quart size jar and thin it down to water consistancy with DNA then dip, and rotate. A quick brush sanding between coats should solve your problems.