Sanding is the time consuming part of any repair. It also determines the quality of the repair. Too much sanding and you will damage the fiberglass, which would force you to add a fiberglass patch to the inside of the kayak, doubleing the amount of time for the repair. Not enough sanding and the repair will break out. Any stress lines need to be sanded all the way to the glass, without damaging the fiberglass.
We love the Foredom flex shaft tool. It is a dremel tool on steroid. We have been using ours for over ten years without any mantainance. A dremel tool is the next best way of sanding a small gel coat damaged area. If you don't have either toolthen an 80 grit sand paper and elbow grease will do the job. You can use a disk or palm sander if the damaged area is large enough, but these tools are hard to use for small damaged areas.
When sanding, take as many safety precaution as possible. Wear a resperator, not just a dust mask. Have another person hold a vacuum to the area being sanded. A vacuum will greatly reduce the amount of airborne dust. We also have a shop air filter running when we do any repairs.