I finished coating the inside of my tank last month with Kreem, had rust from the previous owner, and I noticed that the Tank Prep B (final rinse and water remover ) did a great job of rinsing the tank and then evaprorating to where the tank was bone dry inside. The tank is now sitting on a shelf waiting for next spring. I check it every week and haven't noticed any rust re-developing...
I'm not sure this would work for your application but I'd have to think it would be better the WD-40. I would think the WD-40 would attract dirt and rinsing out next spring might prove to be a challenge.
Your other option would be to store the tank full of gas.
If you don't fill it with gas, I would open up the tank as much as possible, take off the tank lid and remove the fuel level sensor from the bottom.
