Re: Rusty tank
I'd trash that tank and get another one. Also I'd report the seller to Ebay if he refuses to take it back. The next time you run into rust inside a tank, even a new one from a kit, Remove the petcock, fill it about 1/4 full of pea gravel, buck shot (bee-bees) or course sand then shake the heck out of it for a while. The abrasive action of whatever you used will remove the rust without resorting to acids. When you're satisfied with the results rinse the tank with hot water making sure to get it clean. If you have access to compressed air then blow dry the inside or put the tank in direct sunlight or near a source of heat to completely dry it. In high humidity areas, like Louisiana, you can't leave the tank empty for too long before rust will again form. Unless you plan to fill it with fuel mix soon keep the cap on and plug the fuel outlet to help keep out the moist air.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"