When I was working for a fledgling start up we did not throw away anything that could be remotely reused ... and that went so far as my used lab glassware. The final clean out rinse was always water and water in lab glass cannot be left to air dry ... it always leaves spot deposits.
The glass has to be crystal clear, clean and no spots or residue. Here is the trick to rid and clean out your tractors fuel tank. Go to Ace Hardware pick up a quart of acetone... same thing as in finger nail polish remover. This stuff is hydroscopic ... it soaks up water and its impurities like a sponge. Used it on a B2150 fuel tank in place. I disconnected the fuel line at the filter, drained all fuel and then took a bore scope to take a look while it was empty.. Looked clean bit it was dotted with condensate water drops on the sides, bottom and top of the tank interior.
I got a used windshield washer pump connected it to the tank's fuel line and the pump's outlet line to a hose (1/4') that circulates to the fill port of the tank. I put 1/2 the acetone in the tank and started the pump to spray down the tank inside surfaces.
OH CRAP! left out the most critical step... grab a cold beer(s) before you start the monotonous task of spraying the inside of the tank with the acetone.
After a few minutes grab the bore scope and look for places you missed. If it looks good drain the water soaked acetone and put the other 1/2 qt. in the tank and repeat the process till the tank is sparkling clean OR you have run out of beer. Drain all the liquid from the tank. disconnect the pump and the clear the tank of any acetone residual. Clearing the acetone is the easiest part of this process because it is extremely volatile... just drop a air hose in the tank and let it go for about 30 minutes... then check for any remaining liquid... if no liquid you are done.
For me, the best way to minimize the formation of water condensate is to fill the fuel tank after each use AND when you remove the fuel cap ALWAYS wipe the condensate from the fuel cap gasket...