I have cleaned dirty tanks by just pouring in a bucket full of carefully selected 1/2 inch rocks ( just kidding, any clean gravel ), and agitating the tank. I have done this with the tank dry, but found I had better results when I added solvent.
Another method I have used with good results, is to use a solution of TSP, the same stuff used to clean concrete floors, also used by the radiator shop. It was also used in the engine shops to degrease engine blocks before rebuilding them. It is also used to remove paint. This takes less time with heat, but I would not do this unless the tank was completely submerged
When I worked on construction in the early 60's, ( before I got an edgication ) I used muriatic acid to wash brick walls on newly constructed buildings. I don't remember the strength of the product but it was an excellent cleaner. The only protection we had was the 5 ft of brush handle.
My first choice if I was going to use an acid, would be to see how expensive phosphoric acid is, and use that instead. It is commonly sold as Metal prep for etching metal before priming.
Bob
