Don't put fibreglass immediately on top of the foam though. This technique is often used in scratch building submarines. Put different sections (cut to the largest diameter of the section) on a central stick. Then sand or cut to shape. Before fibreglassing they do put something aluminum foil or some sort of tape over the shape, otherwise the resin eats or melts the foam while settling. Not entirely sure what they do/did, but looking at their technique will bring you more advice I'm sure. For cutting the foam they often use a melting wire. Melts its way through rather than actual cutting, gives a smooth finish.
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