How to Repair Gelcoat: Step-by-Step Guide

To repair gelcoat, clean the damage, sand and bevel it, fill it with gelcoat catalyzed at 1.5 to 2% MEKP, seal the surface so it cures, then wet sand and polish it flush. Most scratches, chips and cracks on a fiberglass boat can be fixed this way at home, start to finish, with the right materials.

Figure out what kind of damage you have

The repair depends on how deep the damage goes:

• Surface scratches: marks that stay within the gelcoat layer
• Deep scratches: cuts that go all the way through the gelcoat to the glass
• Chips: gelcoat that has broken away
• Cracks: stress fractures in the gelcoat, often called spider cracks
• Gouges: damage that reaches into the fiberglass laminate itself

Light surface scratches will often buff out. Anything past that calls for a real gelcoat repair.

What you need

Repair materials

• A gelcoat repair kit, or gelcoat matched to the boat
• MEKP catalyst
• Wax additive, if your gelcoat is unwaxed
• Cups and sticks for mixing
• Masking tape
• Acetone for cleanup

Prep

• Coarse paper: 80 and 150 grit
• Leveling paper: 220, 400, 600 and 800 grit
• Polishing paper: 1000, 1500 and 2000 grit
• A sanding block
• Clean rags

Finishing

• Rubbing compound
• Polishing compound
• Marine wax
• A buffer or a polishing pad

Step 1: Clean it

1. Wash the repair area with soap and water
2. Wipe it down with acetone to strip wax and other contaminants
3. Let it dry fully

Do not skip this: gelcoat will not stick to a surface that is dirty or waxed.

Step 2: Prep the damage

Scratches and small chips

1. Rough up the damaged spot with 80 grit
2. Feather the edges by sanding outward so the repair blends in gradually
3. Put a slight bevel on the edges to give the gelcoat more to grip
4. Wipe with acetone

Deeper damage

1. Cut away anything loose or broken
2. Where the glass shows, sand it back to clean fiberglass
3. Fill deep gouges with thickened resin first, then put gelcoat over the top

Step 3: Catalyze the gelcoat

1. Pour out the gelcoat you need and then some, since running short partway through is worse than wasting a little
2. Add MEKP at 1.5 to 2% by volume
3. If the gelcoat is unwaxed, add wax additive so the surface can cure
4. Stir well for 2 minutes

Check the color first: try the match somewhere out of sight before you commit. Gelcoat often cures to a slightly different shade than it shows wet.

Step 4: Apply it

Small repairs

1. Tape off around the damage
2. Work the gelcoat in with a brush or spreader
3. Leave it a little proud, since you will sand it flush
4. Cover it with plastic wrap or PVA so the surface cures

Larger areas

1. Go on in thin coats of 10 to 15 mils
2. Let the first coat tack up before the second goes on
3. Build until the repair stands just above the surrounding surface

Step 5: Let it cure

• Initial cure: 2 to 4 hours
• Ready to sand: 4 to 8 hours, with temperature setting the pace
• Fully cured: 24 to 48 hours

Wait until the gelcoat is completely hard before any sanding. Sand it early and it turns into a gummy mess.

Step 6: Level it

1. Begin with 220 grit to bring the repair level with the surface around it
2. Move up through 400, 600 and 800
3. Keep the surface wet as you go
4. Check often, and stop as soon as it is flat and smooth

Back the paper with a sanding block so you do not dig low spots.

Step 7: Bring back the shine

1. Keep wet sanding at 1000, 1500 and 2000 grit
2. Run rubbing compound with a buffer or by hand
3. Follow up with polishing compound
4. Seal it with marine wax

Done right, the repair is invisible or close to it.

When the repair goes wrong

The gelcoat stays soft

• Too little MEKP: brush more catalyzed gelcoat over it
• No wax additive: without it, air keeps the surface tacky
• Too cold: move the work somewhere warmer or put a heat lamp on it

Fish eyes or pinholes

• The surface was contaminated, so clean more thoroughly next time
• Fill them with another thin coat of gelcoat

The color does not match

• Matching color is hard, so think about redoing the whole panel
• Oxidized gelcoat reads lighter than new, so polish the surrounding area first
• Gelcoat suppliers offer professional color matching

Our take

Gelcoat work gets better with practice. Learn on small repairs in places nobody looks, and with patience and the right materials you will get professional results.

If the damage goes into the structure, the job moves past gelcoat. Our guide to fiberglassing over plywood covers bigger repairs, the resin per yard of fiberglass guide helps you plan the layup, and for custom shapes see how to make a fiberglass mold.

We stock gelcoat repair kits and tooling gelcoat, and the rest of your fiberglass supplies are at Boat Suppliers.