I always fuel proof the entire inside of the tank compartment. Usually use 30 minute epoxy (no thinning with anything) by spreading it on as thin as I can then gently heating the entire area with the heat gun. This makes the epoxy very thin and it penetrates every nook and cranny plus cures somewhat faster when heated. Just do not heat it so hot that it bubbles, just enough to make it run easily. I also drill a hole in the lowest part of the fuel compartment to let that inadvertent problem of split line, ruptured tank etc. to have a place to drain and alert me immediately to a problem if and when it occurs. I usually get one of these after a model has been in service a few years and I get careless about replacing a tank or fuel lines. When Hobby Poxy paints were available, that was my favorite fuel proofing item. I understand that Klass Cote does the same job just as well but I have not yet tried their paints.