Fuelproofing the firewall is a serious business.

I make sure I have every hole and access finished, then coat it in 20-minute thin epoxy all over. Since you have to hold it, it is a two-step process. I even coat the inside of the holes before I put in the blind nuts. Load the hole up, then stick a toothpick in there, swish it around, and wipe up the excess. Paint both sides and all the edges, then wipe them down lightly with a paper towel. This makes a super thin coating that is absolutely fuelproof. In the past, I have seen the screw holes be the point of failure because they are not fuel-proofed. BTW, after you put in the blind nuts, give their piercings a dab of epoxy, too.

Details, details.