With a background in industrial and marine maintenance, my experience with this may be worth considering. Aquatapoxy is, in my opinion, a first rate coating for protecting water tanks. It is even possible to make non-structural repairs by combining fiberglass mesh over lacy spots. The best possible preparation is to white metal blast. While less effective in the long run, chipping, needle gunning and grinding will do in a pinch. I do not have any financial interest in Aquatapoxy, just trying to be helpful. http://www.cohesantmaterials.com/Products.aspx

Pacific Locomotive Association has used POR15 to coat the interior of two tank cars (for water service), one locomotive tender, a saddle tank, and pair of side tanks. We are very satisfied with the POR15 product, our two oldest tanks were coated over 20 years ago and the coating is still holding up very well. We plan on coating the interior of our newest acquisition in the next couple months.

Johnathon Kruger
Road Foreman of Engines
Niles Canyon Railway