I've been told that steel
boats rust from the inside out, and I think it is true. You are right, it is all about prep and if you can sandblast or get all the rust out, the finishes work
well. In the real world where you can get at all the rust there is hope. You need to seal off the oxygen and close the pores. In Europe
they have a lot of barges and because of the cargos can't use paint or a coating to finish off the holds so they use this wax/oil stuff. It goes on thin, doesn't smell too much, almost smells like molasses and dries to a nice dark finish. It coats the rust and doesn't allow the water to penetrate. It saved my boat and I have to thank the old Dutch captain
who recommended it to me. They sell it on the fuel
barges and it comes in gallon jugs. What? don't have a local fuel barge, well you are in luck, I think Toplicht from Germany
will send you some. - https://www.toplicht.de/en/shop/yach...oel-hansalux-u
I recoat every year or two and slosh it up in areas that I can't reach any other way.