If you have a stuck Master Lock or padlock and are feeling like taking the bolt cutters to it try this first.
Put your safety glasses on and give the lock a few firm taps on both sides of the case with your hammer. Do the same on the top of the curved shank.
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With your safety glasses on and spray pressurized air down the lock shank and into any holes on the top part. Repeat for the key hole, sticking the nozzle as far as you can get into the lock. Give it a couple of taps with the hammer and repeat.
Spray WD-40. LPS or Liquid Wrench down the shaft and up into the keyhole. Give it a couple more taps with the hammer. Insert the key and work it from side to side while compressing the lock. With combination locks keep rolling the numbered tumblers around and around and then set them on the proper combination and try to open.