I use DAP Fast N Final lightweight spackling to fill voids in foam. Works great and can be sanded to smooth it out. I wet my fingers to spread it. That helps smooth it out as well.
ORIGINAL: critterhunter
While on that subject, does anybody know a good way to smooth over missing chunks of foam when you apply the tape? My best idea yet is to use gorilla glue to fill and smooth in any voids and tape over.