ok I'll get some Fusion Self-Etch Primer. Shall I use a large can of tank sealer this time? Although the can I had should have been big enough, maybe the surface area was to large?

The area in indented, so will fill with plastic padding or chemical metal before reshaping. Hope the other tins don't cost too much. So far it has worked out an expensive solution.

Yes, get some Fusion Self-Etch Primer and a larger can of the Tank Sealer to make sure you have enough.

what size can of Fusion Self-Etch Primer do i need, and how is it used?

1 Pint should be fine. We have had customer pour it in, roll the Fusion around on the inside of the tank so the entire inside of the tank is coated. Allow to cure over night. Then apply another coat of the Tank Sealer.