I've made up some bases using pieces of foamcore as rocks. The foamcore has a thin cardboard coating but the cut sides have exposed foam. I was just wondering if I have to seal the foam parts before spraying them with black primer. If so, what is the quickest/easiest way to seal the small areas of exposed foam? Would just brush-painting the foam parts with good old chaos black be the best way?

I've heard (seeking confirmation here) that thinning down some PVA glue and brushing a thin layer over the foam will seal it nicely. Try it on a spare piece of foam and then spray it as a test.

PVA glue does indeed work when covering foam. Just water it down and apply a thin coat to the foam.

Yep, PVA or white glue does the trick.

Sometimes called Elmers or wood glue.

PVA glue FTW!

I’ve used latex pray paint as well and it works great. But Wal-Mart black primer is .99c and black latex pray paint is around $8.00 a can.