I just removed the stickers from Frequent Sea and do it a little differently.
I use a heat gun (a little hotter than a air dryer--and has two heats).
The major difference is that I then use a plastic spatchla or spreader (for bondo or filler). A razor blade can cut the gel coat--but the plastic spatchla will get under the letters (and I was able to peel these off with my finders after lifting a corner--no potential damage to the gel coat).
I also use Goof Off, Goo Off etc--and then Finessit II to cmpound the boat and bring the color equal to where the letters prevented fade, then two coats fo Colinite fleet wax. The photos of the boat I don't believe you can even see a ghost of the numbers or sticker.