My tests include polyester resin, polyurethane, silicone (both cheap caulking and expensive additive RTV) and hotglue.
Polyester resin is cheap and nasty smelling, and sets hard. I found it din't "bite" into the foam really well, so later it either cracked where it was thin, or peeled off the foam where it was thick enough to be self supporting.
Polyurethane is more expensive (like 3 times the price) and does bit into the foam. I stuff I have is also flexible, so it bends with the foam as well. It helps make foam joins more rigid and can even blend and fill in seams.
RTV silicone doesn't really bite into the foam, but it makes an awesome skin over the top that is really flexible and can replicate skin really nice. If I had known how well thios worked earlier, I would have used it for my "hammerhead" project.
Caulking silicone is hard to work right out of the tube. It is an adhesive, so it will stick to everything. You can smooth it with acetone or mix it in a jar at 10 parts silicone :1 part acetone to make a thick pour-able paste. This tends to lose some of the adhesive qualities, but makes it much easier to work. Once the acetone evaporates, you are left with a rubber skin over the part. Whilst it is much cheaper than RTV, the acetone will increase the costs and pound for pound, it gets costly.
Hotglue is the cheapest stuff to coat foam, but I have no idea what possessed me to do this. I melted a heap of hot glue sticks in a pot and applied the melted plastic over the foam. It bites in, but also peel up and because of its low melt point, not very stable in hotter environments.
