Why small deck scratches deserve a proper repair

A catamaran gives you acres of wide, flat deck, and that generosity comes with a price: there is nowhere for a scratch to hide. A dropped winch handle, chain rattling across the foredeck, a fender squashed against the hull, a tender dragged over the bridgedeck — all of it leaves its mark, and on a broad expanse of white gelcoat you will see every one of them from the pontoon.

There is more to it than vanity. Gelcoat is your boat's first line of defence against water and ultraviolet light. Once a scratch breaks the surface, dirt, salt and algae settle into it, the edges crumble, and the damage widens. On a cored deck, a path for water is the last thing you want. A scratch that takes twenty minutes to fill today can become a weekend of fairing and drying out in a season or two.

Assess the damage before you open a tube

Run a fingernail across the scratch. If it glides over, you are dealing with a scuff and a polish will lift it. If your nail catches, it needs filling. Look closely at the bottom of the scratch — if you can see dark strands, woven texture or grey filler, you have gone through the gelcoat and into the laminate beneath.

• Surface scuff: polish only, no filler required.
• Shallow scratch: filler flush to the surface, straightforward repair.
• Deep scratch or gouge: filler built up in layers, possibly several sessions.
• Crazing or spider cracks: not a scratch at all — these are stress cracks and need grinding back before filling, or they will reappear within weeks.

Wash the area with fresh water, then wipe with acetone and let it dry thoroughly. Never attempt a repair over a damp or contaminated surface, and give yourself a dry, shaded day with the temperature somewhere between 15°C and 25°C. Direct sun on a dark deck in August will make your mixture go off in your hand.

Match the colour before you mix anything

This is the step most owners rush, and it is the one that decides whether the repair disappears or announces itself forever. Start by polishing a patch of deck about 30cm across next to the scratch. Gelcoat oxidises and dulls over the years, so unprotected surrounding surface reads lighter than the true colour underneath. Match to the polished area, not the weathered one.

Wet the polished patch with a drop of water — this simulates the finished, waxed look far better than a dry sample. If you have the original colour code from the builder, use it as your starting point. Otherwise, take a small pot of white gelcoat and add pigment paste a pinhead at a time: tiny amounts of yellow, brown or black until it disappears against the sample. Test your mix on the underside of a locker lid or inside a cockpit locker before it goes anywhere near the deck. Pure white is almost never the right answer.

Clean and prepare the scratch properly

Wipe the whole area with a dewaxing solvent or acetone to remove polish, wax and silicone. Silicone contamination is the enemy of a good bond, so keep silicone-based dressings well away from the repair. Use the point of a sharpened nail or the back of a scalpel blade to lift out loose gelcoat crumbs and any grit sitting in the scratch. Feather the hard edges with 400 to 600 grit wet paper so the new material has something to key into, then blow the dust out and wipe again.

Mask either side of the scratch with good-quality tape if you are working near a moulding line, a hatch surround or non-slip panels. On a catamaran deck, most scratches sit close to something you do not want to sand.

Build the fill in thin layers

Small deck scratches rarely need more than a few grams of material. Mix a small batch of gelcoat paste with hardener at around two per cent by volume in cool conditions, one per cent in warm weather, and stir it thoroughly with a clean stick. Work in the shade and keep your batch small — you want roughly ten minutes of working time, not thirty.

Apply with a fine artist's brush or the tip of a mixing stick, pressing the material into the bottom of the scratch rather than painting it across the top. Slightly overfill, then cover the repair with a piece of acetate or cellophane taped down flat. Gelcoat cures air-inhibited, so that film excludes oxygen and gives you a hard, fully cured surface instead of a sticky one. Add the next layer once the previous one has gone firm but still slightly tacky. Two or three thin layers will always beat one thick one, which will shrink, sag and crack.

Sanding and polishing to blend the finish

Once the repair has cured properly, usually overnight in UK ambient temperatures, remove the film and start flattening. Use a small sanding block, never your fingertips, and work wet through 800, 1200 and 1500 grit with a drop of washing-up liquid in the water. Keep the block flat and check constantly — you are aiming to bring the fill level with the surrounding surface, not to carve a hollow around it.

When the surface is uniformly dull and flat, switch to a cutting compound on a soft pad, then a finer finishing polish, and finally a coat of wax to seal it. If your repair runs across a moulded non-slip panel, apply the fill with a fine brush to keep it inside the scratch and sand sparingly so you do not flatten the pattern. A light pass with a small brass brush after polishing will lift the texture back up and keep the grip safe underfoot.