You'll see lots in the threads about rusty tanks.
I had a two phase problem. My honda has two tanks in one. One holds oil for an injector.
I bought a kit called Kreem.
If you use this, you should take the tank off.
You need to drain the tank and degrease it. I use lots of hot hot water and strong detergents. I didn't want to further complicate the leak between the tanks. If you have lots of rust you need to make sure you don't creat leaks. I used a pint of bee-bees. They did the job and were a little more gentle than nuts and bolts would have been.
Once your tank is clean, as rust free as possible and oil free, you use a acid that comes with the kit. you need to scale down the directions to a smaller tank. This cleans the metal and makes the rust left into a black coating after 4 or so hours. Once this is done and you empty the tank, you wrince it out with hot hot hot water. You drain the tank and add this liquid that's really nasty (do outside). This evaporates and takes the left over water with it.
The last part is this white epoxy. You add it to a sealed tank and roll the tank around to cote the inside.
Its works very well but you need to follow the instructions word for word.