I used muriatic acid diluted 1:1 or 2:1 (water to acid) and a chunk of chain rattling inside to clean the heavy rust and descale before doing the eastwood kit. Works well, but will rust again quickly - and it is water based so needs to be flushed with water, then acetone which will bind the water and evaporate it, then you blow it out with compressed air and seal it right away before it rusts. The eastwood kit came with a neutralizing phosphate metal wash of some sort that also provided a bit of rust inhibitor to let you do it over two days if you needed.
Don't know if fiberglass resin works or not, but I know that motorcycle gas tanks made of fiberglass are not fuel resistant and need to be sealed with the Kreem sealer, so this would likely indicate 'NOT' to that. Not that flexible either so I'm not sure how long it might last....
my 2 cents.
Steve