The first step in the repair is to remove the damaged area and any excess resin. On a boat with a gel coat you will need to grind the gel coat off completely around the affected area. Your instinct will be to preserve all the original material possible. Though it is hard to see the size of the working area increase and this new horrible looking area that you are creating, it is absolutely necessary to remove any loose fibers or layers that are coming free. One basic rule of thumb is that if you can rip or tear it with your finger you need to go ahead and grind it off.

In the case of the picture below there was no gel coat on this boat but there was the previous patch to remove.

Use low grit sandpaper or a dremel tool to sand away any broken fibers. You must take off all the gel coat near the crack and scuff the surrounding gel coat significantly enough for the new layers to bond. The ideal bond is fiber touching fiber. Resin touching resin or fiber touching resin is not as effective as fiber touching fiber.

It is absolutely essential that you never sand fiberglass without some lung protection. If you do not have a respirator or a particle mask at least wrap something around your mouth. When you are done sanding turn a fan on and leave the area for a few minutes until the dust settles.

Then you must wipe the area with "Acetone" to take away the dust and clean any contaminates off the surface. Acetone can be bought at the hardware store in the paint section. It is really nasty stuff and you should keep your contact with it to the minimum. After wiping do not touch the area with your fingers. The oil on your hands is a contaminate that will inhibit the bond.