What Counts as a Small Chip

Fibreglass panels take knocks, and most of them leave the same calling card: a small chip or gouge where the gelcoat has cracked away and left the laminate beneath exposed. Somewhere under about 25 mm across and 3 mm deep, a chip is a straightforward workshop job. You can usually repair it at home with a bench, decent light and patience. Anything larger, anything that has delaminated or splintered the glass fabric, or anything on a structural part such as a hull below the waterline or a load-bearing bracket, belongs with a laminator who can judge how much material has been lost.

The repair principle is simple. You clean the damage, taper the edges so there is a shallow dish for the new material to key into, then build up layers of compatible resin and glass until the patch sits just proud of the surrounding surface. Sand it flushing, blend the finish, and the chip disappears.

Cleaning and Assessing the Damage

Start by washing the area with warm water and a little detergent to lift dirt, wax and silicone polish. Rinse thoroughly, then wipe with acetone or an epoxy panel wipe and let it flash off completely. Contamination is the single biggest cause of a repair that lifts or blisters six months later.

With the surface clean, look closely at the edges. Press around the chip with a thumbnail or a blunt scraper. Any gelcoat that flexes, crumbles or sounds hollow is already detached and must come off. Work outwards until you reach material that is firmly bonded.

• Exposed laminate: dry, chalky fibres need to be cut back until you see solid glass.
• White or milky patches: these usually mean water has wicked into the laminate. Dry the area with gentle heat for several hours before continuing.
• Dark or soft material: suspect rot or contamination underneath. Dig it out rather than sealing it in.
• Masking: tape off a border at least 50 mm wider than the repair to protect the surrounding finish.

If the chip goes right through the panel, tape a stiff backing plate behind it — a piece of acetate sheet or waxed card works well — so the resin has something to sit against.

Tapering the Edges

A straight-sided hole gives the new material nothing to grip. You need a taper, roughly eight to ten times wider than the depth of the damage. A 2 mm deep chip therefore wants a dish about 20 mm across. This spreads the load and gives each layer of glass something to overlap onto.

Use a rotary tool with a small abrasive drum, or a coarse disc by hand, and grind the edges back at a shallow angle. Feather the gelcoat so it thins to nothing at the outer boundary. Keep the tool moving, keep the pressure light, and stop frequently to check the profile with a fingertip — you want a smooth saucer, not a crater with a lip.

Finish the taper with 80-grit, then 120-grit, to give the surface a mechanical key. Blow or brush out the dust, wipe with acetone, and let it dry. From here on, handle the area as little as possible.

Building Up the Repair

Match the resin to the original laminate. Polyester or vinylester resin for most boat and automotive panels; epoxy if you know the panel was laid up with epoxy, or if you want the strongest bond and are prepared to finish it differently. Never mix systems — epoxy over polyester is generally fine, polyester over cured epoxy is not. Always check the ratio on the tin and measure by weight where possible.

Work in a warm, dry space, ideally 18–25 °C. Cold slows the cure and lets resin drain out of the glass; damp air will cloud an epoxy finish.

• First layer: brush a thin coat of catalysed resin into the taper to prime the surface.
• Chopped strand mat: for polyester work, tear — never cut — small discs of mat so the edges are feathered. Tearing gives a soft overlap that blends invisibly.
• Layering: build up two or three discs, each one slightly smaller than the last, stippling with a brush rather than dragging it. Work out air bubbles as you go.
• Build proud: leave the patch 1–2 mm above the surrounding surface. Shrinkage during cure will bring it down.
• Cover: lay peel ply or a piece of acetate over the wet patch and press it flat. This starves off excess resin and reduces sanding later.

Let it cure fully — longer than the tin suggests if the workshop is cool. Twenty-four hours is a sensible minimum for polyester, and epoxy likes a warm post-cure if the panel will see sunlight.

Sanding Flush and Blending

Once cured, remove the peel ply and check the profile. Any low spots get another mix of resin and mat; don't try to fill them with gelcoat alone, as it will shrink.

Sand the patch back level with a flat block, not your fingers, which will dish the surface. Start at 120-grit, move to 240, then 400. Stop and check the shape against the surrounding panel by running a straight edge across it. When the patch is level, switch to wet-and-dry paper — 600, then 800, then 1200 — keeping the paper wet and flushing away the slurry so it doesn't clog.

Finish with a cutting compound on a foam pad, then polish. If the panel is gelcoated, a thin coat of matching gelcoat over the sanded patch, flatted and polished the same way, will restore the colour and give the repair a seamless edge. Take your time at the last stage; the difference between a visible scar and an invisible repair is almost always in the final sanding and polishing, not the laying up.