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Neil,
Even thought the major stresses/loading of you car will be carried by a steel frame, The long term integrity of the body cannot be overlooked.
You can think of a 'glass-foam-'glass structure as similar to an I-beam. All bending loads are resisted in the two 'glass skins. All the foam does is maintain separation between the skins which handle tension, compression and torsion.
The integrity of the structure is greatly compromised when either skin is broken or dented/kinked. While a relatively thin skin may be sufficient to handle pure compression and tensions loads, the skins have to be substantially thicker to provide puncture/tearing and or denting. The each skin has to be thick enough to resist puncturing/tearing on it's own. There is essentially no contribution to puncture/tearing by the foam.
Cracks/tears in the external skin are especially troublesome. In the short term the strength/integrity of the structure is compromised. There will be localized points of high stress which will lead directly to slow continuing tearing due flexing and vibration. Even if the skin does not tear further, it is almost a certainty that there will be an area where the skin and foam are no longer connected.
Any crack, tear or puncture, no matter how small, that allows water to penetrate through either skin has to be prevented. Once water gets into the structure it essentially can't be removed. As as result, the structure gets heavier over time. By itself, water will cause delamination of the 'glass-foam-'glass structure. Over time, small amounts of water will travel throughout the structure. When exposed to freezing temps it will turn to ice which will pry the laminate apart. In high temps, it will vaporize, turn to steam, and delaminate the structure. (Temps in the "micro-climate" at the surface of boat hulls painted dark colors on sunny days can exceed 250 degF. That's why boats are usually painted white.)
I think that you should be thinking of skin thicknesses of at least 1/16 in. and perhaps even 1/8 in. on the outside.
Books about cedar strip canoe/boat building can give you a preview of methods to get relatively consistent resin application in hand built laminations. To get a full=strength chemical bond between different layers of epoxy/cloth, the following layer has to be applied before the earlier one is fully cured… just a little bit sticky is fine. It's going to be a real challenge to do the inside. You might even end up cutting the body into upper/lower or left/right halves to get full/free access to work.
I am not saying that a 'glass-foam-'glass laminated structure won't or can't work. It surely can, but it has to be done very well and the very last thing you want are skins that are too thin to handle not only the routine expected loads, but considerable additional thickness, just in case.
Rokeby
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