If your fiberglass craft has a pit, a gouge or a small crack in the hull, a good way to effectively repair the problem is to use a mixture of fumed silica -- often known by the brand name Cab-O-Sil -- and gelcoat, which creates a thick putty. This thick putty is great for filling in surface defects, but it is quite a bit thicker than regular gelcoat resin, which may be a consideration when fixing hairline cracks. This putty can repair damage in just one coat, but is often more noticeable as the finished surface tends to have many pinholes. These pinholes are not water permeable, however.
Put on your work gloves, safety glasses and respirator. Pour the color-matched gelcoat resin into the paint cup in a well-ventilated area or outdoors. Add the Cab-O-Sil slowly and stir with the wooden applicator. Stop adding the Cab-O-Sil once the consistency of the mixture is similar to peanut butter. Store the Cab-O-Sil and stir the Cab-O-Sil and gelcoat resin mixture until it is uniform. This should take no more than a minute.
Apply the mixture to the surface defect using the wooden applicator. Use the plastic squeegee to wipe over the mixture and smooth out the surface, as well as to remove any excess mixture. It is okay if the surface is not completely flat, as the mixture will reduce in size slightly as it dries. If there is too much of the Cab-O-Sil and gelcoat mixture, you will sand it down later.
Seal the repair in sealing film and allow 24 hours to dry. Remove the film and check that the Cab-O-Sil and gelcoat mixture is dry. If the mixture is still tacky, replace the sealing film and allow to dry for at least another eight hours.
Use the roughest grit sandpaper to sand the surface of the Cab-O-Sil and gelcoat mixture until it is level with the surface of the fiberglass. Use progressively finer grit sandpaper to smooth out the repair until the surface has a dull shine.
Apply the rubbing compound to the clean cloth. Rub the repair area in a circular motion until it matches the finish of the rest of the fiberglass surface.
