Your tank just "flash rusted". Once your tank is cleaned of all rust, and then you dry it out, there is nothing stopping the air from oxidizing the metal. You need to do one of two things: coat it with a gas tank liner, or immediately fill it with fuel. You have to keep the air off of the surface of the metal or it will rust again. The good news is that the flash rust that is there now should just be a very thin layer that should be easy to remove again with vinegar.
- Frank
1984 200ES Big Red
1985 350X (x2)
1986 350X
1986 250SX
1984 Auto-X
1984 ATC70
1985 ATC70