Lining a tank

I am puzzled. I took the sender unit off the gas tank today and shone a flashlight in. It looks new. Suck my head under the car and there was a bare spot where the undercoating didn't get to due to the tank.

Can a gas tank last 36 years ? ..or do you think the orginal owner relaced it before it was sold..I do know the last 2 PO's didn't do any work on the car.

Good news for me.

We have never had good reading on how full the tank is. It usually measures just over half to 1/4 where it is empty. The bobber doesn't look like it leaksed or water logged so could the problem be just how hard the little runner connects with the metal strip ? Perhaps just a little pressure to bend it in tighter ?

Thanks
ian.