I would get some Kilz either in a spray can or quart can and coat the surface with Kilz to seal the wood. Once dry, I would buy some rubberized undercoating from an auto parts store and spray it well. I find the rubberized undercoating does a good job and holds up for years. It will also seal the gaps where the wood meets the metal framing.
Don & Mary
2014 Newmar Dutch Star - All Electric - 450 ISL
2016 GMC Canyon 4WD Crew Cab