There is nothing on the Frosted Detail page about it's waterproofness? Is frosted Detail or Ultra Detail water tight? I also have a related question - Is one able to say, coat the inside of WSF with a resin of some type that would make it watertight?
I don't know if this helps, but in my last test I was able to coat wsf with marine grade epoxy resin with great results. WSF just soaks that stuff up, so a few thin coats were necessary but it worked out really well. Obviously it will make your model much more rigid. I have no experience with FD so unfortunately I can't comment there.