I'm a Caswell Sealer virgin so I hope my first time is a good one.
Your right.....I really don't know if it was a sealer but it coated the entire tank.....which led me to believe it was......which along with you I never heard of it back then.
What I've been doing is using lacquer thinner and placing the tank in various positions as to help loosen it so I could pull it off. But it seems to have caused rust everywhere so I will have to deal with that prior to the Caswell. There are some areas way in the front that will have the old sealer in it forever as it just seems impossible to clean entirely.
This was how clean I got it to before I had to use the lacquer thinner method to loosen things up over time......now its like i put saltwater in it.