I use muratic acid. It is a very powerfull acid when used full strength. I use it all the time to refurbish parts. It will strip rust off like crazy, but you have to be carefull. Let something tihn set in it too long, and you'll have nothing left. I left a bracket in a bucket and forgot about it for 2 days. I didnt have much to show for when I remebered. /ubbthreads/images/graemlins/banghead.gif
But like I say, as for plastics, the acid dosent harm it, hell its sold in plastic jugs.
Just a side note, I have even cleaned up lock cylinders with it, but it has to be done very quickly so as not to eat aluminum in a mater of seconds. I just have a bucket of water next to the bucket of acid to rinse them imediately.
NOTE: when dealing with acid for purpose's like I mentioned, ALWAYS, ALWAYS have a couple buckets of water to rince your parts and YOURSELF!