hi,
there are several options, you can firstly apply a couple of layers of latex or thinned down Pva glue with water. Then fibreglass can be laid over and worked on or even if you actually thin down some water based clay to a wet slip then give the whole thing thinnest coast ever and I mean not far off dirty water thin, let it dry and rub off any built up areas, its just to give enough of a dusty/matt surface (literally as if the surface is dusty, the clay should be present at all other than having dullened/dirtied the syrface of the foam.) that you can then actually build a couple of thinned layers of plaster or even better jesmonite on by just brushing over, this can then be sanded much easier and safer and cheaper (plaster) than the fibrglass option, it'll just never be quite as durable. And then paint and finish however you like.
The clay tip is something i found out by accident when trying to do something similar myself. All the clean pieces of foam I was coating ready to paint, the jesmonite was just running off but where bits of clay had dried and been removed before this it was holding. ~I guess the pour layer of the clay is just enough to dry out that beauty coat.
much like how plaster moulds almost instantly grip to latex.
hope that might help.