[rotorwrench]

In aviation, a thinned form of what I've always referred to as Pro-Seal is used for sealing wet wing tanks. It is a rubber like material when cured. The metal would have to be very clean and free of any contaminants from corrosion to water. They don't make the stuff thin enough for a slosh type treatment so I doubt is would be a viable alternative and yes, it is very high priced. It's sprayed or brushed on but large aircraft tanks are big enough to climb into to do repairs. A thankless job that never payed any worker enough to do.

Like any form a sealer. They don't last forever. Some last longer than others, The bad thing about the pro-seal is that it doesn't come off with chemicals. It has to be scraped off.

Unless the restorer gas the equipment and skills to fabricate a new tank then we are stuck with de-rusting and sloshing in the usual way. Some tanks can be opened up and re-welded but not all of them are worth it. The Model A tanks are the worst to cut open and re-weld. The rest are easy by comparison. No one has ever reproduced them so there is not a lot of choice.