Fuel Filler

The way you fuel up can have a lot to do with blow back. If you stick the nozzle deep into the filler neck, maybe even mashing it down on the neck, you've limited the amount of air that can be displaced as the fuel gets pumped in. Stick the nozzle in the filler neck an inch or two and against one side, clearly leaving plenty of room for air to escape as you fill up and start filling slowly at first. Air has to get out as fuel goes in. A vent is required, though some tanks use the cap and filler neck as the vent (crappy idea, though cheap for the manufacture).

No commercially available tanks have fillers that go to the bottom of the tank. They are welded or formed (both plastic and metal) to the top of the tank.