Todd Meyer's Fiberglass Tips
Foam/fiberglass composite is, pound for pound, an order of magnitude stronger than steel - when it is properly done. My turret has withstood blows from tree limbs that would have easily crushed a 1/4" plywood shell. When I feel like showing off for the crowd at events, I slam it with a heavy ball-peen hammer to demonstrate its durability.
If you are using epoxy resin, Styrofoam will work. DON'T use Styrofoam if you plan on using polyester resin to wet out your fiberglass cloth or matte - the solvents in polyester will dissolve any styrene-based material.
Instead, use the yellow polyisocyanurate foam insulation board - it usually comes in 1/2 inch-thick 4x8 sheets. The most common variety comes with an aluminum foil facing on both sides, but it is also manufactured without foil facing or with easy-to-peel-off "impact-resistant" fiber facing.
Don't use the foil-faced variety unless you absolutely have to - the foil must be peeled off before the polyester and fiberglass will stick to the board, and peeling the foil off SUCKS. The sharp edges of the freshly stripped-off foil cut like razors. You WILL be bloody before you finish your first sheet.
Talk one of the Hardware Drones into ordering some of the hi-impact fiber-faced variety, or find a specialty firm that carries the unfaced sheet. The theatre and film industries use a lot of this stuff for building sets, as do some sign makers and industrial designers.
Cut out your panels with a razor knife or saber saw and tack them together with hot-melt glue and masking tape. Fillet the inside corners with auto body putty and lay up a strip of matte over the fillet. Take some coarse grit sandpaper and radius all the outside corners once the inside corner lay-up has hardened off. Filling all inside corners and radiusing all outside corners will insure that the fiberglass will lay directly against the foam at all points. Trapped air bubbles between the foam and the fiberglass ARE THE ENEMY!!!
Get a GOOD RESPIRATOR WITH ORGANIC VAPOR CARTRIDGES, a roll of 1 1/2 ounce fiberglass matte, some polyester resin, MEK catalyst, plastic drop cloths, some acetone, rubber gloves, an empty coffee can for mixing, a stir stick, a metal-handled scissors and some cheap china bristle brushes. Tear (don't cut) some of the matte into a pile of rectangles of manageable size.
Paint the foam with a light coat of polyester resin (don't forget to catalyze it first!). Wet out a sheet of the matte over a scrap board and lay the wet matte over the freshly-coated foam surface, tapping the matte down onto the foam to push out any trapped air. Repeat. Keep the scissors handy for cutting any wrinkles you can't get to lay down (usually around corners).
One complete layer of fiberglass inside and out should be enough - the foam is unbelievably strong IF and ONLY IF you sheath BOTH SIDES in fiberglass. Core weight-bearing areas with exterior-grade plywood instead of foam.
Clean tools with acetone before the resin hardens on them.
The reason you tear matte instead of cutting it is that the jagged torn edges mesh with each other
better. If you are using woven glass cloth or roving instead of matte, you cut it with a scissors - woven glass doesn't tear very well. Glass cloth looks and feels like white silk, while roving is coarser like burlap. There are also lightweight alternatives like S-cloth or E-cloth - your best bet on the East Coast is to find someone who builds racing sailboats, those guys know everything there is to know about lightweight composite construction.
If you want to make something ULTRA light and you have a ton of money,
use Kevlar cloth and brag afterwards that your turret is made with
Kevlar composite armor.
This is a picture of the inside of the Gryphonians tank Stuart Little that I took at the 2002 Spplat attack. You can easily see the blocks of polyisocyanurate foam that was used for it's construction. I seem to remember that it is coated with fiberglass on the inside as well but I can't tell in this picture. I'm pretty sure that fiberglass becomes transparent when you add the resin.