Not a fan of ANY tank coating. Melt the remnants of the old failed coating out with MEK, acetone, aircraft paint stripper or whatever. Clean ALL rust with either the electrolysis method, oxalic acid or one of those expensive rust removers.
If there are a few minor pinholes. Find them by watching where your rinse water leaks out. Poke it through in the center. Then use a 1/4" drill bit or countersink tool to make a concave depression and fill with solder.
I did this with a badly rusted Pinto tank, soldered up 10 or so pinholes with a solder gun and it's still holding.
Coating the Inside of tank just makes a mess unless it's done to aircraft FAA standards. (edited)