



How to coat polystyrene insulation foam
TLDR: what to cover a polystyrene insulation foam with in order to protect the surface from dents and scratches.
Hi everyone. I have an old 1970s caravan that I am currently converting into a studio. I bought the caravan from a builder who was using it as an on-site tea room. When I bought the caravan it was completely bare bones which I prefer. It needs a lot of work, but the weather has started to turn in Melbourne and I'm addressing the most urgent things at the moment:
The previous owner had installed a temporary type of ceiling insulation, I think. It's a tiled foam made out of polystyrene. This month it has been absolutely pouring and the roof has held up well, considering it's just foam covering up the bare bones behind it.
What I've done for the last week is quickly silicone any gaps and paint bitumen waterproofing on the actual foam, just for a really quick protection. When I was accidentally touching it with a tool, it would scratch into the foam. I know that the foam will probably need to be replaced and completely redone.
I've been trying to hold off ripping the foam out, because a) I don't know what's behind it, and if there's any severe structural or water damage that would need urgent work b) with the constant rain, I can't risk having an open ceiling while I completely redo it.
However, I've noticed that there is a blue covering, like a masking tape or thin plastic, that's starting to peel off of the foam. The foam is in perfect condition underneath. I just would like to know- if I should peel off this tape and if I can do any kind of paintable membrane or render that would protect the surface of the foam and harden it in the meantime.
I think that a joint compound would be too brittle for the flexible foam. I would use something like a waterproof base coat over the - is that a better idea? I can provide a list of products I'm looking at in the comments.