Rubberized undercoating

I'm sure someone here will have some suggestions.

Bit of background: I'm commencing a build of an enclosed 6x4 trailer into something more user friendly for when my family and I go camping. I want to build a set of pull-out drawers to store items in similar to the pics below.

The trailer has a wooden plywood floor. I've pulled it out, and will cut a new piece, and want to protect the underside of the new floor from moisture. Are there any products that would form a rubber coating over the underside so I don't have to worry about my handiwork getting ruined over time? Can it be applied upside-down (with the floor in place) or would I have to paint it on with the floor out, and then carefully install it into the trailer? Appreciate any suggestions.

Undercoating is a bit of a conundrum on steel so I think it would be less effective on wood.  Once it gets compromised by the first pebble pulling a bit of it off, it lets water in behind it and holds it there.  I think using undercoat on wood is probably not going to give you the protection you're looking for.

Look up West Systems epoxy.  It isn't what I'd call cheap, but it is the cream of the crop for marine epoxy.  I think a good coat of that soaked in would be the ticket, but not exactly easy or cheap.  A less expensive but less durable option would be a good exterior alkyd or oil paint like a quart of Rustoleum.  Similar results could be had with a good deck stain.

If your build means that you can't get to the floor to replace it easily, I might look into more permanent materials; Trex T&G deck boards, PVC fascia boards, or some other non-rottable thing.

So perhaps some sort of plastic/pvc-coated metal sheet or galvanized sheet under the floor might be a better option? What about the stuff they spray in truck beds - once a metal sheet has been put in place first?

edwardh80 said:

So perhaps some sort of plastic/pvc-coated metal sheet or galvanized sheet under the floor might be a better option? What about the stuff they spray in truck beds - once a metal sheet has been put in place first?

A sheet of AL could be wrapped around the whole floor. Something like a steel wrapped door.

Buy Marine plywood, and coat it with West Epoxy as suggested earlier.  They build boats out of this stuff.  It will protect your floor.

My dad has been building campers for quite some time and he uses the tar paint made for where wood fence posts go in the ground from tractor supply and his floors do not rot