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BOILING water
Personally, I'd pass a length of dental floss/string etc across the back of the plate, and pull from one side to the other with a gentlem sawing motion.
Any residue comes off with WD40: spray, rub it in to avoid runs, leave for 30 mins, wipe off.
Alcazar
Just done this on mine. Tried the dental floss but it wasn't having any of it. I ended up sliding a paint scraper up behind the plate to cut through the pads - much easier . Then just a bit of WD40 left to soak in and rub off the remains with a cloth. Don't forget to wash it off afterwards before putting the new pads on!