gssixgun is right a lot of rust removers and bathroom cleaners contain phosphoric acid. Not as many as years ago. When the big anti-phosphates craze hit a lot of brands changed their formula. Some of the hardcore brands kept the phosphoric acid (the ones that still really work). I don't have any here at the moment to check, but the stuff does work well so I do suspect that CLR still contains phosphoric acid.

Phosphoric acid is great at removing rust, gun bluing etc. but if you leave it to long it will eat any pitting even deeper and eventually will leave a rough charcoal looking layer on the steel. Phosphoric acid is also the main ingredient in a lot of metal paint prep solutions. Along with getting rid of any oils or waxes on the metal, what it does is "bite" the surface of the steel leaving a rougher surface for the paint to mechanically bond to.

The thing with chemical cleaning is that you can't pinpoint where it works. It will work the entire surface it comes in contact with not just the rusty spots. To clean with it in a limited and localized way would probably be OK but I would not recommend soaking.

JMHO,

Ray