In my days of flying glow airplanes, where the firewall gets a little "slimy" from fuel / oil, I had good success just using some covering to protect the firewall. Not really sure why the manufacturers always leave the firewall as bare wood. You can coat it with epoxy but a single piece of coverings works just as well.

Covering works well to protect the firewall on gas airplanes too. Here is a photo of my Aero-Works 29% Edge firewall. I have been flying this plane for 4 seasons with no issue. The carb spits a little onto the firewall but I just wipe it off.

SunDevilPilot