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Contents
Gelcoat is your boat's skin: a pigmented polyester resin layer, only a few hundredths of an inch thick, that shields the fiberglass laminate from water and sun. A chip left unrepaired exposes the fiberglass, soaks up moisture and gets bigger every season. The good news is that most small damage can be fixed in a weekend with patience and the right materials.
Diagnosis: what kind of damage is it
Before buying anything, work out what you are dealing with. Run a fingernail across it: if it doesn't catch, polishing is enough; if it catches but no fiberglass shows, you need to fill it; if you see white glass strands or the area flexes, it is a lamination job.
| Damage | How to spot it | Repair | Difficulty |
|---|---|---|---|
| Surface scratches | A nail barely feels them | Rubbing compound; fine wet sanding if needed | Low |
| Deep scratches | Nail catches, but no fiberglass visible | Gelcoat fill and sanding | Medium |
| Chips and gouges | Gelcoat missing, laminate sound | Catalyzed gelcoat or gelcoat paste | Medium |
| Star cracks | Radiating cracks, often around hardware | V-groove, fill and fix the flexing | Medium-high |
| Impact with damaged laminate | Fibers showing or soft spot | New laminate, then gelcoat | High |
| Blisters below the waterline | Bubbles that weep acidic liquid when pierced | Osmosis treatment | Professional |
What materials you need
Catalyzed gelcoat
This is the original material: pigmented polyester resin that cures when you add catalyst (MEKP). It is the best choice for chips and deep scratches above the waterline. It comes in two versions: unwaxed, for intermediate layers (it stays tacky when exposed to air), and waxed or surfacing agent, for the final coat, which cures at the surface.
Gelcoat paste
Thickened gelcoat that doesn't run on vertical surfaces. It is very handy for chips on topsides and the transom, and some formulas are low-styrene with less odor. It is the easiest option for a first repair.
Polyester or epoxy filler
Used to fill deep voids before the gelcoat goes on. If you will finish with gelcoat, the filler must be polyester: polyester does not bond reliably to cured epoxy. Epoxy resists water better and is best kept for below the waterline, under bottom paint, or when you plan to finish with paint.
Supporting supplies
- Acetone for degreasing.
- Plastic spreaders and graduated cups or a precision scale.
- Wet-or-dry sandpaper from 320 to 2000 grit and a hard sanding block.
- Clear plastic film or smooth tape to seal the repair from air.
- Nitrile gloves, goggles and a respirator: catalyst is corrosive.
- Pigment to tune the color if needed.
How to repair a chip step by step
1. Working conditions
Work between 60 and 77 °F (15 to 25 °C), in the shade and with a dry hull. Below 60 °F gelcoat cures poorly; in strong heat it kicks too fast and you run out of working time.
2. Clean and degrease
Wash the area and wipe it with acetone to remove wax, silicone and grease. A trace of wax is enough to make a repair peel off months later.
3. Open up and bevel
With a small rotary burr or a utility knife, open the chip into a V shape and remove all loose or cracked gelcoat down to sound material. On a crack, follow it to its end. Beveled edges give the filler more to grip.
4. Sand for adhesion
Sand the inside of the hole and a small margin around it with 80 to 120 grit. Vacuum the dust and degrease again.
5. Mix the gelcoat
Weigh the gelcoat and add catalyst at the manufacturer's ratio, usually 1 to 3 percent by weight, with about 2 percent at 68 °F (20 °C). Stir slowly, without whipping, to avoid trapping air. Mix small batches: you have 10 to 20 minutes of working time.
Watch out: eyeballing the catalyst is the most common mistake. Too little and it won't cure; too much and it cures fast and turns brittle.
6. Fill
Apply with a spreader, pressing to avoid bubbles, and leave the fill slightly proud of the surface: gelcoat shrinks as it cures and is sanded flush afterward. In deep holes, build up in two or three layers.
7. Seal from air and cure
If the gelcoat is not waxed, cover the repair with plastic film or smooth tape so it cures at the surface. Leave it at least 24 hours, longer in cool weather.
8. Wet sanding
With a hard block and soapy water, level the repair and work up through the grits. The block prevents dishing around the repair.
| Stage | Grit | Goal |
|---|---|---|
| Level | 320-400 | Remove the excess down to hull level |
| Refine | 600-800 | Erase the previous grit's scratches |
| Pre-polish | 1000-1500 | Leave an even matte finish |
| Finish | 2000 or finer | Prepare for compound |
9. Polish and protect
Buff with compound until the gloss returns and finish with wax. Buying criteria are in our guide to gelcoat polish and compound.
Matching the color
White gelcoat is not one single white: there are several shades, and older boats drift toward yellow. If you can, get the color code from the builder. Test the shade on a piece of tape stuck to the hull and, before deciding, polish a nearby patch to see the true color without oxidation. Pigment pastes let you tune the shade a little at a time; write down your proportions in case you need to repeat it.
If a slight difference remains, polishing the whole side hides it well. For the prep clean, any of the hull cleaners on the market will do.
Star cracks and osmosis: when it stops being DIY
Star cracks usually come from impacts or from the laminate flexing, for example around a stanchion base that works under load. Open them into a V to the end of the crack and fill. But if flex is the cause, the crack will return: reinforce from the inside with a backing plate and check the bedding of the hardware with a good marine sealant.
Blisters below the waterline that weep a vinegar-smelling liquid point to osmosis. A few can be opened, dried and filled with epoxy; widespread osmosis means peeling the gelcoat, drying the hull for weeks and applying an epoxy barrier coat system. That is boatyard work. The same goes for an impact with damaged fibers or soft areas: the problem is structural and you should talk to a professional.
Common gelcoat repair mistakes
- Skipping the degrease: hull wax makes the repair peel off.
- Catalyzing by eye: always weigh.
- Leaving the fill low: sanding leaves a dip you can see in raking light.
- Sanding without a block: waves appear around the repair.
- Gelcoat over epoxy: adhesion is unreliable.
- Covering a crack that keeps reopening without fixing its cause.
- Working in cold or damp conditions: the cure stalls and the surface stays tacky.
Tip: do a test on a scrap of fiberglass or waxed cardboard with the same gelcoat and catalyst. If it doesn't cure at the surface, you know you need wax additive or better air isolation.
How to prevent gelcoat damage
- Use enough boat fenders, placed well and at the height of the dock.
- Wax twice a year: protected gelcoat cracks and stains less.
- Protect the bow roller and anchor area, where many chips begin.
- Check hardware bases: a star crack warns you that the fitting is working.
- Use winterizing season to do repairs at leisure and at the right temperature.
It helps to keep a decent tool kit aboard: a burr, spreaders and sanding blocks are what people miss most.
Recommended gear for this guide
Frequently asked questions
Can I repair gelcoat myself?
Yes. Scratches, chips and small dings that don't reach the laminate are within reach of any patient DIYer. Structural damage, large areas and osmosis blisters are best left to a professional.
How much catalyst do I add to gelcoat?
Usually 1 to 3 percent of the gelcoat's weight, with 2 percent typical at around 68 °F (20 °C). Use less in heat and a little more in the cold, always within the manufacturer's range. Weigh it: eyeballing makes it easy to overdo.
Why does my gelcoat stay tacky after curing?
Because unwaxed gelcoat does not cure fully when exposed to air. Fix it by using waxed gelcoat for the final coat, or by covering the repair with plastic film, smooth tape or PVA while it cures.
Can I put gelcoat over epoxy filler?
It isn't recommended, because polyester does not bond reliably to cured epoxy. If you will finish with gelcoat, fill with polyester products; if you use epoxy, finish with a compatible paint.
How do I match gelcoat color?
Get the color code from the builder if you can, test the shade on a piece of tape and polish the neighboring area first to see the true color. Pigment pastes let you adjust a little at a time.