Most of the video reviews I found for tank sealers are for motorcycle tanks. The $73 (Amazon price) POR-15 tank sealer kit gets great reviews, but it isn't cost effective for a Corvair tank . 3 of the 4 quarts included are metal prep & cleaner, and there's only one quart of actual sealer included. As mentioned, I used 2 quarts of Red Kote ($50 total) on the first coat and only one quart came out. That surely does not mean that one quart would give full coverage, you need lots of extra fluid to get into all those internal nooks & crannies. A second quart of POR-15 sealer costs $35, so by then you're well past half the cost of a new tank. Red Kote was the only option that made sense.
I briefly considered muriatic acid to clean & derust the tank. A few people have videos about this, but too many unknowns inside a tank that you're doing blind, so I stayed with safer Evapo-Rust. $15/gallon at Pep Boys online.