John T wrote:
You'll want to avoid using polyester resins on your foam, unless you
want your tank to melt!

I can see how this would be a problem. The foam would crush on impact
with a paintball, the fiberglass would eventually fail from the
dissortion of the foam underneath it.

maybe you could bond some aluminium skin to the foam. It'll dent like
crazy, but it might last longer!

Actually, the s-glass embedded in epoxy will perform better than the
aluminum. Thus the name "composite". You are combining a product, glass,
with a high tensile strength with something that has poor tensile
strength, epoxy. Then you bond that to something that has a high degree
of stiffness (foam). The idea is that when yo
u try to deform the foam/fiberglass composite, the high tensile strength
covering tries to distribute its load out over the foam. The higher the
tensile strength coating, the larger the area of load distribution.