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David, When welding on fuel tanks of any kind, always do a detergent cleaning first. Remove the tank and drain all fuel and then use a hot water solution with clothes detergent (like in an orange box). Use about 1/2 cup detergent per gallon of water. Fill the tank about halfway and slosh around thoroughly for at least 5 minutes. I've also used small gravel to help break rust pockets loose. Rinse with hot water several times. If you have to weld on the tank, the exhaust gas provides a gas-free, inert environment that won't support combustion. If possible, feed the gas through the tank drain and let it vent out the fill pipe. Seal the "supply" fittings/hose tightly--allow the gas to vent freely while welding. This method provides insurance for any "pockets" of vapor that escaped the cleaning process. If you have access to CO-2, this gas also provides a means to "inert" the tank. There are others available at your local welding supply company. I've seen "brave" folks welding on diesel fuel storage tanks that were full, but I would never recommend welding on any tank without gas-freeing it first. If you really want to be safe, you can also get a marine chemist to "sniff" the tank to test for volatile fumes prior to welding.
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