How much alcohol do I need to add to epoxy in order to use it for fuel proofing? Do you just mix it by eye, or is there a good ratio to shoot for?

I generally use apprx 1/4 to 1/3 alcohol to 3/4 to 2/3 epoxy. I don't think it is an exact science, but the more alcohol you use, the more difficult it is to keep it from running and the longer it takes for it to dry. You can fuelproof with straight epoxy, you just have to be mindful not to get it too thick (save weight) and to work quick, get it on before it sets up.

I usually mix 50/50, or until it flows smoothly with a brush.

You can also heat it with a hair dryer or heat gun as you brush it on. It will turn liquid and penetrate the wood, but it also begins curing quicker. . .