Dave, I would recommend you bite the bullet and put in a new tank. There are plenty of threads you can search as this has been discussed at length many times. You won't have to worry about the rust issue again. I replaced my tank myself (230SL). It was relatively simple. Just remember to take all necessary precautions when dealing with gas and fumes. Drain all fuel from the tank, disconnect and remove fuel sender, disconnect fuel lines, disconnect the breather tubes (you can access these by removing the round cover at the front of the trunk to the right), remove gas cap and rubber surround, and unscrew the 4 screws holding the tank in place. I don't think I forgot anything. You might have to work it a bit to get it out. You should not have to remove the muffler, although you may have to loosen the screws holding it up so you can move it a bit if it is very close to the tank. I'm not especially mechanically inclined so I am amazed I actually did it myself.
Good luck.
