Thanks For the info on that,If it's leaking from the seam then I would sand the tank down, degrease it, rinse the inside/let it dry out, and then solder the entire seam. When I was a heavy equipment mechanic, we used to braze leaks, but I think solder would be easier to work with and would be fine for a small tank like this. You'll need to make sure you use solder designed for use with steel though.
Any type of degreaser and then follow it with hot water. You just need to make sure it's bone dry before putting oil and fuel back into it. You can dry it out by putting it in an an oven at a low setting. I just degreased and cleaned my whole saw and then threw it in the dishwasher on a hot water and heated dry cycle, and it came out great. Just follow that up with another cycle (with the dishwasher empty) with bleach...unless you want to piss if your wifeany ideas on what to flush the tank out with