Ok, I coated a piece of scrap foam last night with DeckOver and just subjected it to some stress testing. The coating is slightly flexible and is more impact resistant than uncoated foam, but I don't think it would qualify as a hard coat. An impact with a hammer that broke the surface of the uncoated foam just left a slight dimple on the coated side.
I just put thin coat on them but the surface felt harder than the foam and I think it had a little give to it. I still have some left so I'll put some on a scrap piece of foam and see just how durable it really is.
How about a thin-set mortar like this. I haven't tried it as a foam coating but I don't see why it wouldn't work. You could even mix it with some exterior house paint to make a waterproof monster mud-like coating.
For me the Halloween season starts November 1st. That's when I start taking down that years decorations and planning what I'm going to do next year. I work on new props and decorations all year long as time, money and inspiration permits. It usually kicks into high gear around August 1st.