On Sunday, May 5, 2013 8:57:31 PM UTC-5, woodchucker wrote:
Sounds like that might not be a bad idea. how do I neutralize the muriatic acid in the tank after, just water or baking soda...

Flush with water. It shouldn't take much acid (1-2 cups) to rinse/slosh the bottom of the tank, where there would likely be the most rust. Concrete is cleaned with muratic acid and simply rinsed with water. The old saying is to add acid to water, to prevent a splattering reaction when mixing the two. In this situation, adding water to the acid (to neutralize it), inside the tank, shouldn't be a problem, plus it would be difficult to pour out the acid, first, into a safe container without having it pour irradically from the petcock hole. Further, pour the mix into a 5 gal bucket of water, to further dilute it.

Will that clean off most of the loose rust?

Maybe/yes, to some extent. I've used it to assist in removing rust on stuff at home(shop) and used it on (our past) boat dock surfaces. Since you can't readily see inside the tank, we don't know if there is significant rust in there, or not. Muratic acid is more readily available, I think, and will help loosen at least some rust without having to scrub, since you can't readily scrub the inside. Any potential loose rust may block the petcock, in the future, so removing the most you can is preferrable. Using acid is the easiest way, IMO, hence my entertaining the idea of using the acid as a "rinse" or possible (assistant) rust remover.

Summer jobs, during college days, I worked on a dredge boat/barge along the Gulf coast. There were times when we'd clean parts of the deck for repainting. An acid was used to help remove (or treat?) the rusted areas. I don't recall what acid was used. We'd slosh it on, do some minimal scrubbing, then rinse with water. On tough or deeply rusted areas, we'd use a grinder to remove the rust, best we could. Apparently the acid "treatment" did some kind of good.

Muratic acid is pretty potent. If spilled on the outside of the tank, it will likely peel the paint off, fast.

Sonny