Skip Links

Topside Gelcoat Repair

Repairing scrapes in your hull isn't as difficult as you'd think. Here's how to make a seamless repair on vertical surfaces.

My Grand Banks 32, Seaglass, is in good condition for her age. But over the course of her 30-year lifespan, she's suffered a few bumps and bruises, all now successfully repaired. What was in less-than-stellar shape, however, were the sides of the flybridge that once had a plethora of antennas and other equipment, since removed, leaving behind some ugly holes and previous weak attempts at repair.

Repairing gelcoat so that it blends into the original is not difficult given the right mindset, tools, and materials. But making repairs to vertical surfaces presents some unique challenges. Top of the list is that you're fighting gravity. But there are several ways to overcome this problem, and the exact approach depends on the size of the repair.

Safety Talk

Protect yourself, especially from dust and fumes. Use common sense, and read and understand all the safety instructions that come with your tools and materials. The gelcoat catalyst is especially noxious; avoid breathing in the fumes. Wear a respirator and disposible gloves. If you get gelcoat on your skin, wash it off right away with soap and water.

In the article "Easy Gelcoat Repairs", I outlined the basic steps needed to perform an invisible repair to gelcoat on the cabin top. Undertaking a minor repair on a horizontal surface, such as the one outlined in that article, is a great way to gain confidence and hone your skills before moving on to a repair that is more conspicuous, like on the hull topsides or flybridge.

Technical Support

Degree Of Difficulty: Moderate to difficult

Tools and Materials:
  • Respirator
  • Latex or nitrile gloves
  • Mixing sticks, cups
  • Plastic spreader
  • Foam brush
  • Waterproof abrasive paper in various grits from 120 to 600
  • Sanding block
  • Random-orbit sander & extension cord, if needed
  • Acetone
  • Gelcoat (plus tinting, if needed)
  • Paper towels/lint-free rags
  • Mylar sheet
  • Compound and polish
  • 3" plastic shipping tape (3M Scotch or similar)

Time: About 3 hours plus curing.

Cost: About $50 for materials, not including gelcoat & tinting.

Filling holes with epoxy filler

1. The sides of the flybridge are structurally sound but looking rough. The previous owner had hastily filled holes with epoxy filler and left them. The first thing I need to do before I start the repair is to carefully wash the area I'm working on.

Sanding down repair

2. I start by sanding down the previous poor repair to bring it flush with the surrounding surface. I used a random-orbit sander with 180 grit abrasive paper. You can sand it by hand, but it will take longer.

Clean area with acetone

3. Wipe down the area with acetone to clean off any residue, such as wax, that would interfere with the repair.

Force filler into holes with putty knife

4. Fill any holes. If they're small or shallow, you can use some thickened gelcoat.The ones on my boat are one-quarter inch diameter, so I use epoxy filler, which has the consistency of peanut butter. Force the filler into the holes using a putty knife. Leave the filler a little proud of the surrounding surface so that you have something to sand down level with the existing gelcoat, but don't overdo this. A further application of filler is often preferable to overfilling any hole or repair, especially if it's large, as this avoids sagging or unnecessary sanding of the very hard epoxy once cured.

Sand filled holes flush

5. Leave the filler to cure overnight, then use a random-orbit sander with 120-grit paper to sand the filled holes flush and fair with the surrounding gelcoat surface. If needed, apply more filler. (Not shown: If, after sanding, there are still some small holes, fill them using some two-part all-purpose surface filler formulated explicitly for fiberglass repairs, such as Interlux Interfill 833. After mixing following the supplied instructions, apply the filler with a plastic spreader, applying no more than necessary to minimize the amount of sanding needed.)

Abrasive paper wrapped around block

6. Before you apply gelcoat, get the surface perfectly smooth. Although it's tempting to use a power sander for this, doing so can create ridges in the surface. Instead, sand by hand using 120-grit abrasive paper wrapped around a block, pausing frequently to run your fingertips over the repairs to check progress.

Switch to 320-grit wet/dry paper used wet. With the paper wrapped around a block, frequently dip it into a bucket of clean water, which keeps the paper clean and lubricates the abrasive, making for faster work and a smoother finish. Apply moderate pressure to the abrasive paper until all scratches from the coarser paper have disappeared.

Wipe down surface with cloth

7. Once the surface is smooth and free of imperfections, wipe down the surface with a lint-free cloth dampened with acetone. Turn the cloth frequently and wipe, in one direction only, to remove any surface contamination that might affect the gelcoat that you apply.

Mixing gelcoat

8. Mix the gelcoat according to the manufacturer's instructions. How much is needed depends on the size of the repair. It's better to mix up a little too much than not enough. Work swiftly. Once catalyzed, you only have about 10 minutes of working time before it starts to harden.


Shining a bright light at a low angle across the repair area will help you spot any dips and bumps in the finish.

Getting a good color match is one thing that causes anxiety among newbies. The best approach is to purchase gelcoat from your boat manufacturer so you'll know it will be a perfect match. If this isn't possible, buy clear gelcoat and add some tinting pigments a little at a time until you achieve the right match.

Applying gelcoat

9. To apply gelcoat, you have two choices: brush or spray. The easiest method for most folks is to brush on the gelcoat, which is what we are showing here. Use a good quality brush. Cheap "chip"-style brushes, often used for polyester resins and other tasks, shed hairs, marring the finish. A better choice is either the same style and quality of brush you would use for, say, painting the trim of a house, or a foam brush that can simply be tossed once the job is complete.

For many repairs, a single coat is often sufficient, but for deep gouges and other significant damage, the gelcoat sometimes needs a more substantial coat. Either apply multiple thin coats or a Mylar covering (available from stationery stores), as I did, to prevent sagging. (See "Get The Gelcoat To Stay Put" below.)

Removing mylar covering

10. After leaving the gelcoat to fully cure overnight, remove the Mylar covering.

Wet sand the repair

11. Wet sand the repair to seamlessly blend the new gelcoat into the original. Use 240-grit followed by 400-grit abrasive before finishing off with 600-grit. To prevent ridges, wrap the paper around a block.

As in the prep stage, check the repair with your fingertips to feel for any imperfections. Frequently wipe the area using a rag to monitor progress. As an added check, sight across the surface from several angles with the side of your head almost touching the surface. Any undulations are easiest to spot this way.

Compound with wool pad

12. Satisfied with the surface? Then use a fine cutting compound with a wool pad to finish off before applying a good quality wax to protect the surface and prolong the shine.

Get The Gelcoat To Stay Put

In many respects, gelcoat is similar to paint. As it needs to be applied in a somewhat thicker layer than paint, however, it is predisposed to run, sag, and not stay where you want it. There are several things you can do to prevent this from happening.

1. Apply several thinner coats instead of one thick coat. You'll need to lightly wet sand between coats to remove any surface wax and wait for each coat to cure. Building up a decent thickness, especially if you have a deep gouge, could take some time.

2. Thicken the gelcoat. Some experts suggest thickening gelcoat to a paste consistency with the addition of some form of filler powder. WEST system 410 microlight filler powder is one popular option. This helps, but any filler is likely to affect the gelcoat color and can make the surface more porous than the surrounding original gelcoat.

3. Apply some sort of cover to help hold it in position. For small jobs, such as repairs to hairline cracks, a piece of plastic shipping tape, of the type mentioned earlier in this article, is a great option. After applying the gelcoat, stretch a length of tape across the repair sticking it down to the adjacent sound gelcoat. For larger repairs, clear Mylar film, available from office supply stores, is an excellent choice as it can be lightly squeegeed into the still-wet gelcoat giving a smooth finish. By removing the air, there are no problems with any waxy surface.

After curing, both Mylar and tape can be easily removed.

Related Articles


Click to explore related articles

how to diy install and repair exterior


Mark Corke

Contributing Editor, BoatUS Magazine

A marine surveyor and holder of RYA Yachtmaster Ocean certification, BoatUS Magazine contributing editor Mark Corke is one of our DIY gurus, creating easy-to-follow how-to articles and videos. Mark has built five boats himself (both power and sail), has been an experienced editor at several top boating magazines (including former associate editor of BoatUS Magazine), worked for the BBC, written four DIY books, skippered two round-the-world yachts, and holds the Guinness World Record for the fastest there-and-back crossing of the English Channel — in a kayak! He and his wife have a Grand Banks 32.