What I suggested is two part epoxy. It doesn’t really slosh. The viscosity while warm is consistent with honey. If you allow it to get a little thick on the bottom it may add structural integrity. It’s tough stuff. As to the prep, Caswell say to remove the loose stuff and line it. I have anecdotal evidence that it works like that. As for me, I removed all evidence of rust with phosphoric acid before the epoxy went in. I have lined two tanks with it. One has over 3,000 miles since lining, the other tank is currently living comfortably in the house. I’m confident in the stuff.
