What to do with the seams between foamboards?

[jwb]

Provided the foamboard is attached well enough to the plywood, I don't think this is an issue as far as the behavior of the material is concerned, but it's certainly a visual issue. You can fill the cracks with whatever basic scenery material you're using -- plaster, Celluclay, spackle, whatever. I would do that before ballasting, since the ballast costs money (unless you have a good free source of fine grit) , and the cracks will eat it up.