fill the tank with soap and water, and make sure you have burped it to make sure it is COMPLETELY full of water, no trapped air pockets. While the tank is full of the soap and water mixture start cutting.
If the tank is full of residue from oil, you might pressure wash the inside using a cleaning solution, (purple power or some of the same alternative) and then rinse out, then fill it with the water and soap solution and start cutting.
My dad is 50 year welder (He is 84 now), and he has never hesitated to weld on gas tanks, or propane tanks as long as it was slam full of water. But he goes throgh meticulous methods to make sure that there are NO AIR POCKETS left inside..
Hope this helps.