3M 5200 is not removeable without a special remover solvent they sell just for that purpose, and even then it is a real pain. It also cures under water. I had to remove a transducer from the hull of my boat once and it ripped a chunk of Fiberglas off when it finally released.
If you use a product like this I highly suggest using 4200 as it IS removable. Since you used the fast cure version rather than regular 5200 you probably did not achieve a good bond or you would not have gotten it off without damage.
