What to do with the seams between foamboards?

[videobruce]

Not really a "design" question, but using 1/2" rigid foamboard over 3/8" plywood with seams staggered so they are not in line with the plywood seams.

Can/should the seams between the foamboards be covered over like drywall seams? If not, isn't movement a issue and the resulting cracks?

[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.

[Sumpter250]

I always try to have relatively straight/vertical edges on the foamboard, and use siliconized acrylic caulk on the mating edges, as well as on the foam/plywood contact.
Unless the finished area is supposed to be "flat", I cover the foam with plaster cloth. on "flat" areas, any gap in the seam is filled and sanded, before laying the "surface" material.
The visible seam in the lower right hand corner, is the module boundry, you can also see it on the left side, bottom.
The surface of the walk area is very fine black "ballast".