If I is really rusty inside, then you could put some crushed gravel and about 1/4 tank of water inside it and roll it around to loosen up all the rust. Then shake out all the gravel and dry up any water remaining.
Put on a good water trap and filter and you wont have any problems.
Note: unless the tank has flaking rust, just fill it with fuel and keep a good filter on it and you wont have any problems. Tilt the tank so that the end with the hose is at least 2" higher than the opposite end. Most of those tanks have 3/4" drain plug on the low end where you could install a threaded nipple and valve. Remove the plug and put it in the end of the valve and then occasionally remove the plug, open the valve and drain a gallon or so of fuel out to check for water. You could pipe the valve to a lower location for easier operation also.
