While this tank needs some paint attention the real problems lay inside, where bad gas and time had turned the fuel to varnish and metal to iron oxide.
I began the tank rescue by soaking for two days with Liquid Plumber. What came out is shown at left. The acrid smell could be sensed throughout the 'hood, or so it seemed. But that was only the first round. Next up was a four-day soak in Evapo-Rust ($25 per gallon at Harbor Freight). Instead of dark roast coffee, what came out was more of a tea color...and more rust. This tank was in such bad shape that the fuel supply tubes feeding the petcock were clogged solid. I spent an hour with a stiff wire carefully clearing them.
Finally, I thoroughly coat the interior with WD-40 (a water dispersant) by spraying through both tank openings, sloshing around inside, and draining out the bottom. Reassemble all the hardware and the tank is ready for use.