GPW wrote:Finch, that plastic under-floor may seem like a good idea now , but remember any non-permeable material will do a better job holding water IN , than keeping or letting it out ... Water always finds a way in (ask anybody here ) ... the idea is to get the water OUT before it has a chance to soak up into anything

I really appreciate your help. I already treated the floor frame, but with full strength varnish. The thinner I got made it half watery, half a cloudy gel. Maybe I got the wrong stuff? Maybe I didn't use enough thinner? The guy at the HD paint counter said it would work to thin it.
Keeping the water out was only half the purpose. I intend to fill the gap with foam. (I wouldn't buy a cooler that had one side with no insulation... and I don't like cold floors)
Our original plan was to wrap the bottom with canvas(avoiding the black nasty stuff if possible ). Maybe we should revisit that idea. I guess we could use plywood but we have managed to avoid it so far, and I'd like to keep it that way if possible.