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Once the gel is cured, you can begin sanding. This process is fairly straightforward, but there are a few tricks.
Wet-sanding the repair with water speeds cutting and keeps sandpaper from clogging. Wet-dry sandpaper is readily available in grades finer than 300 grit and can be found coarser if you look around. Ideally you’ll have wet-dry sandpaper in 320, 400, 600, 800 and 1000 grit. The 800-grit and finer papers can be found at marine-supply stores. Unless your repair is huge, you’ll only need one sheet of paper per grade.
It helps to use a sanding block of some sort. Rectangular chunks of minicell foam work extremely well. A minicell block will conform to the curved shape of the hull better than one made of wood or rubber.
Start with 100 or 150 grit to remove material. Do the rough shaping with this paper and switch to 220 to remove any deep scratches. Once you’re satisfied with the surface, move to 320. Each progressively finer paper will remove the scratches from the previous grade. Moving through all the grades doesn’t take long, so don’t start with  the finest grade right away. If you jump from 320 grit to 1000 grit, you’ll eventually remove the scratches, but it’ll take forever and use up a lot of sandpaper. Keep in mind that even fine sandpaper will remove some material from the hull. Try to anticipate this as you sand, so that your repair finishes flush with the hull.
Use plenty of water. When you think that you’re finished with one grade of paper, wipe the sanding residue off of the hull with a rag and look at the repair from an angle to get a better view. This will let you get a better look at the finish. You’re looking for obvious scratches and for a uniform texture that indicates that the area has been thoroughly sanded.
Before you move beyond 320 grit paper, take a look at the repair and decide if you’re happy with it. This is the time to apply another layer of gel coat if you need to. This coat should be smooth and thin so that it can be sanded and faired with 320-grit paper before you move on. If you need to add another layer, lightly sand with 220 grit, mask off the repair and repeat the application of gel coat.
If you’ve done your work in the sanding department, the next part will be easy. You’ll be using a buffing bonnet on a drill or grinder to polish the hull. It’s difficult to apply enough “elbow grease” to shine past the 1000-grit paper that you used. Don’t use a car buffer. Most polishers used for cars are too gentle to bring out a deep shine in fiberglass. Using a high-speed polisher will let you buff the hull to a full shine and will blend away the last of the sanding marks.
With the drill and bonnet, you’ll need to use a rubbing compound. There are plenty of rubbing compounds available, but my favorite is 3M Super Duty Rubbing Compound, which is available at marine-supply stores. 3M also makes a product called Finesse-It, which is intended to be used after their rubbing compound to increase the shine. Try the rubbing compound alone first. You can always buff again with the finer compound if you’re unsatisfied with your initial results.
Apply the compound to the hull and go to work with the buffer. Use steady pressure and move the bonnet over the area. Keep the surface of the hull moist. The rubbing compound will dry as you use it, so add more to the hull or dribble a little water on to keep things lubricated. You shouldn’t have to polish for too long before you have a mirror shine. Wipe away the compound from time to time and look at the hull from an angle. If all has gone well, it will be hard to tell the repaired section of the hull from the rest of the boat.
Your first attempt at gel-coat repair may take a little longer than an afternoon, but once you have the hang of it, it’s easy to do. With practice, your results will improve, and you’ll learn your own tricks. Once you’re confident with gel-coat repair, you’ll be less worried about using your boat to its full potential. Remember not to worry—nothing is forever in fiberglass.
Materials List
3M blue masking tape
3M Finesse-It polishing compound (optional)
3M Super Duty Rubbing Compound
Acetone
Paraffin canning wax
Catalyst
Denatured alcohol
Drill or grinder with polishing bonnet
Fiberglass mat
Gel coat
Paper cups
Pigment
Sandpaper 60, 100, 220, 320, 400, 600, 800, 1000 grit
Stirring sticks
Styrene
Waxed polyester resin
Brian Day has been paddling, breaking and repairing sea kayaks since 1992. He lives in Madison, Wisconsin. |
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