Use epoxy on the firewall to fuelproof it. A lot of people thin the epoxy with denatured alcohol to spread it . There is a better way to spread the epoxy. Put a "blob" of (unthinned) epoxy on the firewall and heat it with your heat gun. As it heats it will thin out and you can spread it with a brush. The problem with using epoxy thinned with alcohol is that if you use too much alcohol the epoxy will not harden properly. Also, when you heat the epoxy it will actually penetrate the pores of the wood and will better seal the wood. In addition to the firewall and cowl cheeks you will want to fuelproof the compartment where the fuel tank is located too. If you get the epoxy very thin while applying it you won't have enough extra weight on the plane to worry about.

Hope this helps

Ken