I ended up covering my floor panels with aluminum sheet and FRP (a.k.a. Marlite, milk house board, etc.) depending on location. Excuse the following long story, but it may contain useful lessons learned (at my expense) for other trailer owners.
Since I had to replace the entire floor of my '81 Scamp
13, I decided to use marine plywood. Now, there are lots of different products being sold as "marine plywood", some of which are made from exotic (and probably unsustainably harvested) tropical wood, others from Baltic birch, still others from Douglas fir, etc., using various types of glues, preservatives...just visit some boatbuilding forums and you'll see what I mean. After calling around, I decided to try Home Depot, which, in this area, special orders various types of specialized lumber from a local lumber treating company called Chicago Flameproof. The HD contractor sales guy ordered me two sheets of this so-called 3/4" marine plywood, which I proceeded to cut to shape to fit my Scamp.
Just as Scamp does and just as lots of rebuilders recommend, I coated the bottoms of my panels with a thick layer of polyester resin before glassing them back into the shell. I've used this resin a few times before on some other projects, including some home repair ones and I know how to use it. It cures pretty quickly if mixed correctly. The resin did, in fact, harden where it was applied 1/8" thick, but some thinner sections were still sticky, as were the edges. I decided to check it again the next day just to see what would happen. As it turns out, any resin in contact with this whatever-it-really-was (I think it was also treated with flame retardant) plywood just plain refused to cure! Not only that, but a few places where I had made some marks on the plywood with black permanent marker sort of oozed out, turned into sticky, black slime and soaked deep into the plywood. I removed the coatings of half-cured resin and black slime with a heat gun and a putty knife, all the while uttering ever greater obscenities.
I could have tossed my $65 per sheet plywood and started over with plain stuff, but, more importantly than the money, I had a fair amount of time invested in cutting and shaping those floor panels. I also wanted to install really good underside protection. However, I had to make those panels stick to my resin. What I ended up doing is attaching 1/16" alloy 5052 aluminum (a very corrosion-resistant alloy) to the bottom sides of the panels that need to be glassed in and running about a 2" wide band of the same material around the outer edges of the top sides. Top and bottom pieces were riveted on with 3/16" aluminum pop rivets. Aluminum was glued on with the cheapest clear silicone caulk I could buy. I used silicone because it sticks well to the metal and my mutant plywood - even with heat-gunned half-cured resin still stuck to it - and it was easy to reposition the aluminum. No immediate contact cement grab to worry about. The edges of the aluminum were roughened with an orbital sander and 60 grit paper to (hopefully) help resin adhesion.
The non-glassed-in panels in the drop floor received a less spendy treatment, which was FRP (fiberglass reinforced plastic) panels. This is that obnoxious-looking white pebble-finish stuff you see on the walls of truck stop rest rooms. It was on sale at Menards for about $18 per sheet. I think the FRP panel adhesive cost more than two panels. Since it's glass reinforced, it's fairly tough. It isn't rated for outdoor use because UV from sunlight attacks it, but I'm betting it will be OK on the bottom of a trailer.
I have yet to install these, since I was sidetracked last November by this thing we have here called winter. Just as well, since I ended up having to tear out my Ensolite headliner due to mold and separation - but I'll save that for another post.
Will these treatments work? Who knows? If this works out nicely, I'll definitely write up a post.
I bought my aluminum from Online Metals - good prices, exotic stuff in small quantities, and really, really dumb humor. http://www.onlinemetals.com
Chicago Flameproof really does sell good stuff - just don't use their plywood if you want polyester resin to stick to it. http://www.chicagoflameproof.com/