Short answer
Minor gelcoat damage, such as stone chips, scratches and hairline cracks, you can fix yourself with a gelcoat repair kit: clean, sand out the damage, fill with gelcoat mixed with hardener, cover it so it cures, sand smooth and polish. If the fibreglass underneath is damaged, there is moisture in the laminate or the damage is below the waterline near through-hull fittings, have a boatyard take a look.
Assess the damage
| Damage | DIY? |
|---|---|
| Surface scratches | Yes, polishing is often enough |
| Chips, edge damage | Yes, gelcoat filler |
| Star-shaped hairline cracks | Yes, but look for the cause (stress, fittings) |
| Exposed fibreglass or soft substrate | Boatyard – the laminate needs repair |
| Grounding damage to keel or hull | Boatyard, see running aground |
Step by step
- Clean with acetone to remove wax and grease. Manufacturers stress that old wax will stop the repair from bonding.
- Open up the damage with a small burr or knife into a clean V shape. Bevel the edges.
- Mix the gelcoat and hardener exactly as the kit instructions say. Tint with pigment to match the shade. Old gelcoat has often yellowed.
- Fill slightly proud of the surface. Gelcoat cures poorly in air, so cover it with plastic film or use waxed gelcoat.
- Sand wet in stages, from coarser to progressively finer paper, until the repair is flush.
- Polish with rubbing compound and then fine polish, and finally wax. 3M's procedure for boats follows the same order: sanding, compounding, polishing, protection.
Tips
- Don't work in direct sunlight or cold. Curing is affected by temperature; follow the kit's temperature guidance.
- Mix small batches. The pot life is short.
- Wear a respirator and gloves. Styrene and sanding dust are hazardous to health.
- Below the waterline, the repair often needs epoxy and new antifouling, see antifouling.
Tools and materials
| You'll need | For |
|---|---|
| Gelcoat repair kit | Gelcoat, hardener, pigment |
| Acetone and rags | Degreasing before and between steps |
| Small router or multi-tool | Opening up the damage |
| Filler spreader and plastic film | Filling and covering |
| Wet sandpaper in several grits | Sanding the repair flush |
| Sanding block | Flat surface without dips |
| Rubbing compound, polish and wax | Bringing back the shine |
| Respirator and nitrile gloves | Protection from fumes and dust |
Expect to fill deep damage in two rounds, as gelcoat shrinks slightly as it cures. Always sand with a block, not your fingertips, or you'll get a dip that shows against the light. It's a good idea to test on a hidden area first so you learn how fast the material cures.
Save a jar of the mixed colour for the next repair.
When it's a boatyard job
Large areas, osmosis blisters or structural damage often require lifting, drying and laminating. Request a quote via Marinpedia to compare boatyards in your area. Find boatyards under Marinas & boatyards.
Frequently asked questions
Can I use ordinary plastic filler?
Below the gelcoat layer it's fine, but the surface must be gelcoat or topcoat. Otherwise the filler absorbs water and shows.
Why does the repair turn sticky?
Gelcoat without wax won't cure when exposed to air. Cover it with plastic film or use gelcoat with a wax additive.
How do I match the colour?
Mix pigment in small test batches and let them cure. Wet gelcoat looks different from cured.