Technical article from Interlux
If you have ever dinged or scratched your boat when docking or scratched the topsides, you've probably had to take it to a yard to get repairs made, but you can do the job yourself at a considerable cost savings. This process is not difficult, for example you could easily repair minor cracks in the deck or hull. If the mark is really deep and affects the structure or hull integrity, you should go to a boatyard to get the damaged area fixed properly.
The first task is to chip away any loose filler or gelcoat and then grind the edges of each ding or scratch back. Grinding back is done to get rid of any loose materials and to clean up the damaged area.
Each ding will then need to be partially filled with a layer of filler, use Interfill from Interlux. Interfill is a two-part filler that will provide a new strong bond with the damaged area. The areas around the scratch or ding will also need to be masked off to prevent damage to these areas while doing the repair. Trowel the filler into place with a small plastic spatula and roughly smoothed. If your spatula is too large use a razor knife and cut it to size. After the filler cures, grind back slightly below the surrounding area, and fill the damaged area a second time.
When the second application of Interfill has cured, sand with a long-board using 220-grit sand paper. Then refill any slight hollows to bring the work flush with the hull. This part of the job is tedious, but taking care now pays off at the end. When sanding filler and gelcoat, wear a dust mask or respirator. You do not want to inhale the dust particles.
As the dents or dings are filled use finer, and finer grades of sand paper until the last grade is 320-grit. Having reached a satisfactory finish, mask off the surrounding area and spray or brush on a primer coat of Interlux Epoxy Primekote 404/414 primer. Once dry, the primer can be lightly sanded back using 320- grit wet and dry paper and the residue wiped off with Interlux's Special Thinner 216 solvent. A second coat of primer is then applied and sanded back to give the topcoat something to adhere to. The topcoat of choice for this job is Interlux's two-part polyurethane Perfection, which can be applied with a brush or roller.
If the areas to be brushed are too large use a roller. In cases where the area is very large you may be better off having a boatyard finish it for you. The yard may have to repaint one entire side or the complete hull to get the color to match the original topcoat color.
By using fillers and paints made by the same manufacturer, the entire job is assured of compatibility. If you use products from different manufacturers, you may find that they react with each other, and the finish may be compromised.
Original article from Interlux