While a razor blade can done safely, if they're "cracked" as you say, that will be a VERY tedious job. Another method is using a heat gun to help soften the glue and allow the peeling/pulling to go pretty quickly. But again, with the "cracking", you're constantly starting over. For this case, the best tool I know of is made by 3M. I call it an "eraser" tool - it's about 10, 6" rubber discs all stacked together. Each one is very thin - think about what 10, flexible, CD's would be like. They are clamped together in the center and the clamp/post is inserted into a high speed (a couple thousand RPM's is needed) drill. It works like MAGIC. Google it - you'll like it! Acetone will clean up any remaining glue residue.