Once the gel coat has dried, add new inner masking tape. This tape will get damaged as you sand and you will have to replace it so you don't sand the non-repair area. Put the inner tape 1/8" away from the gel coat to help sand down the gel coat ridge.
As this will be the finished product it is important to sand slowly and follow the contour of the kayak. Start with an 80 grit sand paper and work down to a 220 or even better 300 grit sand paper. Don't start with too high of a grit sand paper as it will take too long to sand.
There is a best time to sand a repair. It is just when the gel coat is totally hard so it will not gum up the sand paper. At this point the new gel coat is softer then the rest of the boat and will sand faster. When you are down to the high grit sand paper you will find that the sand paper will last longer and the sanding will happen quicker. This is usually a couple of hours after the gel coat has been applied, but not waiting overnight.
A hand disk sander works great. They are soft enough help follow the contour of the kayaks, but stiff enough to make the area flat.
The bottom photo shows the repair just before the finished product. At this point the high grit sand paper is used to smooth out the area as much as possible. Move the inner tape a bit further away (1/4") to get rid of the last of the gel coat ridge.