Picked up a 1" Starrett micrometer on ebay for $8 that was rusty and froze. Just wanted to try my new ultrasonic parts cleaner. On the first day I used straight Evapo-Rust...did OK..all rust was gone but still wouldn't open/close. Then I put it back in the ultrasonic cleaner this time with straight apple cider vinegar for half a day...that seemed to clean it up just a little better, did not see any rust anywhere...but still wouldn't open. Used my soldering torch and applied some heat near the thimble end where the locking wedge would be...and it opened right up. It wasn't the rust preventing it from opening but the dried lubricant holding the parts together.
Did some finish work & might still paint it...just a quick gauge for my workbench..have the better units in my tool cabinet.