Owner, Green Bay Packers
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Having seen the pictures, I would forego, for just now, Dog's use of the razor blade. Take a putty knife and put a good sharp edge on it. A belt sander works well for this. If necessary buy a new putty knife, but either way, keep a good sharp edge on it. The sharpness is what is going to keep you from digging in to gel-coat. You can round the edges of the blade ever so gently, but the overall face must remain square.
Take the putty knife and use it as Dog described for the razor blade. And as Giu said, like a chisel, you can use your rubber mallet to rap the handle and with as parallel to the surface as you can get, you should be able to pop sections of this stuff loose. Hopefully, it's epoxy resin and the UV light has already been at work on it, loosening it.
Wait for final clean-up to break out the acetone as it will soften up, temporarily, the gel-coat as well as the resin. It will probably soften the gel-coat faster than it will the resin, too!
Don't be afraid to give the putty knife a good rap with the mallet-it's gonna take some force. I would use about a 2" blade, or less, on the putty knife. You're going to gouge some gel-coat, it'll be darn near impossible to do so on a project this size. Pick up a small can of finishing gelcoat at westmarine and you can repair as you go.
The stuff that has entered the pores of that old gelcoat, you are not going to get out. You can try bleaching it out. Might have some long term success along with the sun's UV rays. You can try the acetone or MEK at this point. Otherwise, it's sand it smooth and paint it. The rest of the gelcoat looks pretty porous too, so painting was probably in your future ultimately, anyways.
I'd say, "have fun", but that would be cruel. It ain't gonna be.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.