Excerpted from Fiberglass Boat Repair Illustrated
Patching a hole above the waterline in a single layer of laminate is relatively simple if the hole is small (such as that left by a screw) and you have access to the back. All you need to do is tape over the back of the hole and fill it with a thickened resin mixture. I prefer epoxy to polyester for this kind of repair because it is easier to thicken and sets up fairly quickly. By using fast-acting hardener when you fill small holes, you can get the repair to set within a few minutes If you have a lot of holes to fill, however, mix only a small amount of epoxy at a time, or it will cure before you are halfway through your project. Despite using thickener and fast-acting hardener, the initial repair is likely to leave a surface dimple when it hardens (due to shrinkage and absorption around the hole edges), in which case you will have to apply a second, surfacing coat to make the repair flush. Finish the repair with color-matched paint.
For more tips and techniques to help you maintain and repair your fiberglass boat, purchase Fiberglass Boat Repairs Illustrated from Practical Sailor.
