Borrowing on from theatre scenery, which in turn has borrowed from industry, the optimum material is likely to be IDENDEN. It is a thixotropic PVA based compound which is sold as fire-protection for lagging etc., but in theatre we use it for scrimming carved foam sculptures and scenery - both to fire protect them and to give them an unbreakable, paintable shell. Being PVA based, you can pre-colour it, or you can paint it afterwards. We would normally lay butter-muslin or similar over the foam, and then stipple this stuff into it, or sometimes just paint straight over the foam. You can create texture, you can water it down, and it dries in the same way as PVA, always remaining flexible.
I always use this technique for my scenery, but I cheat, and instead of using IDENDEN, I use the readymix polyfiller, and put a large quantity of PVA into it, so it becomes a paste. This bulks the PVA out and gives texture, and gives a bomb-proof, crack-proof scenic shell.
Available in white, grey and black if my memory serves..... The stuff isn't cheap, but iitt certainly does a good job.