they make several different things that accomplish the task of fuelproofing i think most are listed under building supplies on tower hobbies.
I always used 30 min epoxy thinned with up to 50% alcohol. works great spreads good and also helps to reinforce the firewall from the front side. if you mix a bit much you can fuelproof/reinforce the backside of the firewall and fuel tank compartment also.

As far as when to fuelproof, that's a decision of your choice. Most of my kits i fuelproof right before covering. if you get a bit of runout from the engine compartment you just sand it down when it dries.