You need to find the root cause as to why it is immersed in so much fuel and oil that it seeps into the wood in the first place. Tank leak? Hole in a fuel line? Loose covering in the exhaust stream? It should not require added sealing beyond a proper 1/8" covering overlap seam and a wipe-off with a rag or paper towel after flying (Windex or other mild cleaner a big plus). You do also want to fuelproof the interior fuselage around the tank and firewall and outside in the engine compartment. Either with thinned epoxy or a canned poly wood finish.