[Standing Room Only]

When you dremel the crack a bit, if the glass underneath is 'white' rather than the dark amber, a good chance the glass itself is cracked. If you just fill in the gelcoat, the cracks will comeback as it flexes. Be careful about using filler where something structural -- like biaxial glass -- should be used. If it is a narrow crack, grind into the glass a bit with the dremel to provide a good seating, and then use the Epoxy and microfiber. If it is a whole network of cracks or worse, then use the grind and glass method. I had a ton on my boat because the previous owner let it beat up on a seawall for a time. Never broke the hull, but I had some star cracks from point impacts at the waterline, as well as gouges in the gelcoat.

One thing about gelcoat is that it flexes at a different rate than glass, and it is more brittle. Just gelcoat, and the cracks will come back. At a minimum, you will need to make the crack wider with a narrow dremel, fill with epoxy/filler, then sand and regelcoat. Anything else is a bandaid and cracks will appear again after awhile. The process can be accelerated if you add too much hardener to the gelcoat -- a fast cure makes it more brittle than you can imagine.