Might you have rust in the tank? If so, there is an inexpensive permanent solution, it's called Kreem. You should be able to find it at any bike shop. It's two parts- one is phosphoric acid, to eat the rust and prepare the tank. The next part is a plastic resin. It's easy to use. Take your tank off, drain it, and remove the pet*ock. Follow the instructions from there.
I used to buy inexpensive bikes that sat around, fix them up, ride them, and sell them. This was my best friend.