I would never consider sealing a tank. You will not be able to surface prep the inside well enough for the sealant to stick and last. It may flake off in due time. Buy a used tank if yours is rusted out, which is hard to belief.

I had a W123 tank from an 83 300D sitting outdoors for 2 years and there was not a bit of rust on it. The tank is well made out of quality steel. It has a coating of lead and tin (called terne) which is highly rust resistant. I have no need for that tank anymore so had it cut open to have a look and there was not a bit of rust inside either. The only place rust can come from IMO is the fuel cap, which unfortunately is not made out of quality materials. I would suggest drain the tank, wash it out and look inside with a borescope and see what you find. If it is indeed rusted out, buy a good used tank.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked