Gave the outside a quick hose-off and wipedown, then rinsed out the inside with water and simple green to try and de-gas it some. Sloshing around a gallon or so of water in this is a good workout.
This is happening of course because the inside seems to be rusting, according to my fuel filter / carb bowls. Visual investigation says yes, but it could be worse.
Next I’ll be rinsing it out again with more simple green and water (to remove more gas residue, as well as loose rust), then will start the process for the POR15 “fuel tank coating” kit (I’ll detail that later). With replacement tanks hard to find and $600+ it’s worth a shot.