Quote:
Originally Posted by Blink
Would the acid enlarge the un-rusty piston bore, thereby ruining the caliper, or would it just go for the rust?
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Chemical type rust removers are usually based on inhibited phosphoric acid. The inhibitor prevents the acid from attacking fresh metal after the rust has been removed/converted to ferric phosphate and such. The acid is certainly not dilute or weak. In fact it's typically around 80% concentrated, so not to be messed with. It will take your eyes out if they get splashed (so wear goggles and neoprene gloves).
As to whether it would attack a clean piston bore in a rusty caliper, I suspect not. Not to any extent that would affect its operation. The metal surface may stain/blacken, but it shouldn't be etched. The product referred to in the above link is only 45% strength, so weaker than the industrial rust removers my old company made, but it should be suitable. Basically follow the instructions to the letter.
An impressive alternative to acids is the electrolytic method which does quite amazing things and in many ways is preferable to chemicals.
There are a few threads here on using a car battery and bucket for de-rusting parts. You can also find videos on YouTube.
EDIT: YouTube videos:
https://www.youtube.com/results?sear...c+rust+removal
TC