What kind of seal? ........an old gas cap seal I'm guessing?
Drain the fuel
Remove the tank
Open the lid
Invert the tank and shake vigorously until the seal comes out
This will also remove the 'loose' rust as well.
If the olde seal is stubborn, get some extra long needle nose pliers or similar and pick it out manually.
Matt.
EDIT: It has just occured to me that you mean 'old seal
er', not seal...... where's my head tonite

In that case, I have no idea. sorry. I'll leave my first answer up for humor value
