Yes, you should definitely fuel proof the engine area and tank compartment if you can get to it. Without fuel proofing fuel and oil will soak the wood in that area and eventually weaken the wood. Thinning as Deadeye instructed is good, but I prefer to apply the epoxy without thinning if possible. If you thin the epoxy too much it will never completely harden and will stay soft. To apply without thinning I mix 30 minute epoxy, use a stick (or your epoxy brush) to apply a good "blob" of epoxy on the area to cover, and then heat it with you heat gun. This will thin it out to where you can spread it out with the epoxy brush. Heating the epoxy will shorten the cure time of the epoxy, so that is why you want to use 30 minute epoxy, if you use 5 minute it will set before you have a chance to spread it out. Using this method will allow you to put a thin coat down that will protect you wood.

I hope this helps.

Ken