Gelcoat repair and prep??

The problem is that gelcoat, to my understanding, that is used in molds is always waxed. So yes you need to sand down to the actual glass if you want a really strong bond.

When I bought my highsider someone tried to glass over the gelcoat on the front keel, I was able to easily peel it up with a fingernail and pulled the whole thing off in one sheet.

 

You need to grind through the gelcoat.
Put the glass onto the old glass.

 

I wouldn't waste my time sanding.
take a grinder to it.

I'd get a regular angle grinder with a flap wheel and hit it up.
Or, a small angle die grinder with the screw on 36 grit sand paper discs.

 

Hold on...the title says gel coat repair. Are you fixing a crack in the boat or are you filling in gouges/scrapes in the gel coat.

There is a big difference in what you need to do.

 

You are correct. a dremel works wonders with stress cracks.

Here is anothe tip to lessen the sanding. Go to staples and get overhead projector transparancies. Get the ones from 3M for high heat (they are thicker and stiffer). Cut a piece that is a little larger than the gouge (example: if the gouge from sanding is 3 inches long cut the piece about 5 inches). Take the gel coat and dribble it in the gouge making sure you put enough in that it will fill the entire area. Now take the piece you cut and gently lay it over the repair area and tape one end down so it won't fall off. Take a flat/straight edge and gently go over the wet gel coat to flatten it out. It will squish the gel coat into the gouge and the extra will be spread out very thin. Tape down the other end and let it dry. Take off the plastic and your almost done. The film, because it is plastic will be semi attractive to the surface of the boat and cling to it to some degree and that is what will cause the gel coat to spread out real thin. Just don't use so much gel coat that it slobbers all over, use just enough to fill the gouge and a very slight extra. After it is dry just do your final sanding preparing it for your final finish.