[vectorit]

I wouldn't spray the wood.
It's 3/4" plywood, it doesn't resonate road noise like sheet metal.
Most of the time, the wood is oiled on the underside from years of leaking running gear spraying it down. But the topside is typically dry, being indoors and covered. I think if you covered it with that stuff it would seal it so much, that the wood may start to do weird things. Such as warp, or bubble your sprayed on material.
I'd just get a thick rubber truck mat for the wood if you plan to put heavy parts on it, if not then you can do something like what I did and that was to just lay down jute padding and rubber under sided carpet. All removable of course, for ease of maintenance.