CONGRATULATIONS! That ,ight be the longest, certainly the thickest patch we've seen. What do the rest of The Usual Visitors thin?
We'd try the methods mentioned above. We've used 40 grit on a random orbital sander. Or an oscillating multitool to remove excess patch. Possibly a foot adze or mini excavator
The trick is to go slow with any method in order to avoid damaging good parts of the hull.
CAUTION:
Move the boat outside if you can, a lot of fiberglass and resin dust/fumes will be created. Wear P95 dust mask, safety glasses, long sleeves, gloves,long pants = cover all skin and go rinse off as soon as you're finished. Stand upwind of dust and fume cloud.
If you go the heat route, have fire extinguishing method nearby, as you can set a boat on fire...ask us how we know...there could be stray bits of interior foam or old, dry wood bits inside that make excellent kindling. For our boat VIPER a wood batten from a previous repair started smoldering, we used a damp towel to cool it off.
The heat gun does not stop at heating just the patch resin, it will soften the hull resin as well...ask us how we know.
You'll be priming and fairing before you know it!
Our tradition, if we spill paint, is to make Sunfish art. We're curious if the folks who bought our Florida place left it?
We (I) spill a lot of paint, sometimes on purpose.
VIPER became our most spoiled restoration, his new Skipper sailed him all over Choctawhatchee Bay, with frequent lunch runs to Okaloosa Island.
Usually not well bonded as said. Heat and whatever works best. Some place might have more bonding then others. The vibrating tool with a putty knife is a good option, but it gets tough around the bend.
I have grinders and a DA electric sander. The DA sander has been my goto lately with 80grit and 120 grit. To me 60 grit is a bit too course and for around the bend and wanting to be surgical and just getting the glass off I would tend towards the 120 grit by machine. But stop short if getting every thing off and hand sand with 80.
For a proper repair you will need to rough the surface to the proper grit and feather the edges.
If the glass is well bonded then you should not be able to remove it with heat. Sand it off is the only way.
Take note of what does come off easy and why. When you go to do repairs make sure you dont repeat those mistakes. That is how I have learned and why I frequently comment on glassing. I see others making those errors that failed on boats I have seen.