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That's interesting, Chuck. I've never looked for phosporic acid. I don't even know where you'd find it.
I haven't used the muriatic acid yet. Subsequent to my post I learned a disquieting fact about using it and leaving it inside your garage/shop when it's outgassing. I guess the fumes will cause everything steel thing within a few feet of the solution to rust up and/or discolor. So, it's imperative you use it outside in an open area. Also, the fumes can damage human lung tissue pretty easily. It's recommended you wear a respirator at all times when using it.
I'm kind of sorry I bought it. One of my neighbors tells me a gallon of white vinegar at Walmart is super cheap, like $2.50-$3.00 or so. I think that's a better way for me to go.
Muriatic acid is basically Hydrochloric acid, so yes, pretty nasty stuff. Depends on the concentration, but it can do bad things to you. Great for etching concrete and removing lime deposits (if the substrate can handle it).
Phosphoric acid is the active ingredient in Naval Jelly and most "met-l-prep" type solutions. It burns through the rust and leaves a phosphate based film on the parts, works quite well and leaves a great tooth on the surface for paint adhesion. It generally prevents flash rust as well. You will also find it in Coke, hence why Coke removes rust (and just being on the acidic side of neutral too).
Vinegar does quite nicely as was stated, and can be cheaply found (except recently it has been sold out in my local stores, probably because it is a good "natural" cleaner.