Gas tanks

Like was said, those aromatic hydrocarbon solvents will cut the organic gums and varnishes, and that said, Methyl Ethyl Ketone (MEK) is my favorite.

But for the safest, cleanest, job, put a cork in the tank, set it on a two buckets or sawhorses, and fill it with some household lye dissolved in water. Lye (Sodium Hydroxide) will emmulsify the organics, and rinse away cleanly with a shot from a pressure washer or even a garden hose. You might want to let it sit over night. Rinsing with hot water is even better, and it will promote drying the tank. Then swish some Phosphoric Acid in it as a rinse, let sit a bit, then rinse again. Get out all the water you can, and then swish a couple of quarts of diesel fuel inside to oil, so it won't rust until it's installed and filled.

If you fill it carefully with the lye water, you'll be able to check it for leaks at the same time. This has worked well for me.

My 2 cents.

Frank-in-Tallahassee
70D // 855

I was at my automotive paint/supply store just today and they had Ospho brand on the shelf. I try to find "The Right Stuff". I think it's a bit less expensive. Another brand you sometimes see at shows is "MetalPrep".

F-I-T

All the things you're mentioning are petroleum derivatives -- similar in composition to paint thinner. Muratic acid is a way different sort of animal.

You should be able to find those solvents at a chemical supply house, printing supply house, or even a paint store or auto parts store. Be careful, like paint thinner, they go POOF very easily.

Steve