Stan--
Hydrochloric is the wrong acid. Use a product called "Naval Jelly". Degrease the tank with strong dishwashing liquid and hot water. Add a four ounce bottle of Naval Jelly and add warm water. Slosh it around to mix the pink goo and water and coat the entire inside. Do it several times over a 10 minute period. THEN add pea gravel or coarse builder's sand and rinse that around several minutes. Rinse with hot water.
Naval Jelly is a phosphoric acid/sulfuric acid and a binder. It leaves a VERY rust resistant finish. On very fine firearms it's called "French Gray" and is usually used in 'Bulino' style engraving work.
Well said sir! I'll expand a little, for those of you who want some background (or maybe because I like to hear myself type...), but you've got the rules dead-on.
<puts on chemist hat>
Muriatic (Hydrochloric) acid belongs to the family of acids made with a halogen (chlorine, fluorine, bromine, iodine). All halogenated acids are very, very bad for metals. Back in my lab chemist days we always avoided halogen acids like the plague. It's not just that they're more dangerous (though, with the exception of HCl they are) than most other acids, they also are just about the only thing that does actual damage to passivated 316 stainless steel, which is the most inert cheaply-machined metal commonly available for lab gear (in my case, stuff like High Performance Liquid Chromatography pumps, tubing, etc).
Another downside to using hydrochloric on a fuel tank is that fuel tanks are often made of zinc plated or galvanized metal. If so, the HCl will decompose into chlorine gas when it gets in contact with the zinc layer until it has eaten all the zinc off the steel. Neither the evolution of poisonous chlorine, nor the removal of all the zinc is usually regarded as a good thing.
We generally used 6N nitric acid for passivation, which works well, but is pretty rough on human beings. Phosphoric is a great all-round choice for cleaning random crap off of metal, and it isn't especially bad for people - doesn't fume much, and the by-products are relatively harmless. Naval jelly is one way to get your H3PO4, the other, easy, over the counter way is with "Bar Keeper's Friend" which is used for scouring stainless sinks, etc. Another convenient way to find it is in paint prep treatments like Ospho
http://www.ospho.com/.
And yes, a phosphate coating on steel is a decent rust-preventative. It's not as good as zinc chromate primer, but it does OK. If you've ever handled a gun with a "parkerized" finish, you've met phosphate-treated steel. The chemistry of parkerizing is more complex (and thus the rust protection better) than just phosphate treating steel, but phosphoric acid isn't a bad place to start, especially if you'll be applying a surface finish afterward. The only way I have been able to make header paint stick to cast-iron headers over the long run is by spraying them with phosphoric/water solution after electrolytic derusting.
