My buddy has an automotive machine shop. One piece of equipment in there is a tumbler set up with steel shot and a compartment that has steel hoops welded to it. You put your hardware in the compartment with the steel shot latch it up turn on the timer for 20 minutes and it rotates the compartment in a bath of cleaning solvent. This is the greatest piece of fastener cleaner I have ever used. After the gunk and rust is off....I take my parts to a plating shop for zinc plating and pickling to normalize the hardware after plating. I have done some bench testing against new grade 5/8 fasteners to check for problems of hydrogen embrittlement and loss of hardening....make sure your plater knows what the heck he is doing if you have fasteners redone that way. I am going to buy or build one of those tumblers if I ever move away.......
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