Sanding and polishing a composite part can be deeply satisfying. You start with a dull, orange-peely surface and end with a deep, glassy finish. But composites are not like steel or timber. The resin and gelcoat are thin, heat-sensitive, and capable of hiding nasty surprises. Work too aggressively and you can burn through in seconds, exposing fibres and turning a small job into a major repair.

Why composites need a careful touch

Most composite surfaces consist of a thin gelcoat or resin-rich layer over structural laminate. That outer layer might be only 0.3–0.5 mm thick. It provides the colour, gloss, and UV protection. Sanding or polishing generates heat, and heat softens resin. Once the resin softens, abrasive grit cuts faster than you expect. A moment of pressure in one spot can burn through the gelcoat and reveal the fibre beneath.

• Heat is the enemy. Friction from sanding or polishing raises surface temperature quickly, especially on dark-coloured parts in direct sun.
• Thin areas are vulnerable. Edges, corners, tight radii, and repaired sections often have less gelcoat or resin than the flat panels.
• Dust is hazardous. Composite dust can contain glass fibre, carbon fibre, and cured resin. It irritates skin, eyes, and lungs, so control it from the start.

Setting up a safe workspace

Before you touch the surface, sort out your protection and dust control. Good habits here make the whole job safer and more enjoyable. Work in a well-ventilated area, ideally with a downdraught table or extraction. If you cannot extract, wet sanding is your best friend because it traps most dust in the slurry.

• Wear nitrile gloves, eye protection, and a suitable respirator. For dry sanding, use at least a P2 or P3 filter; for spraying or heavy dust, go higher.
• Cover exposed skin. Composite dust is itchy and can cause contact dermatitis. Disposable coveralls are worthwhile.
• Use a spray bottle with water and a few drops of washing-up liquid. This acts as a lubricant and keeps the paper from clogging.
• Keep plenty of clean microfibre cloths and paper towels. Grit contamination is the fastest way to ruin a finish.

Working through the grits

Never jump straight to a fine grit to remove a scratch. You will spend forever and generate heat. Instead, progress methodically. Start with the coarsest grit that will remove the defect, then step through each grade. For most finishing work, that means starting at P600 or P800. Only use P400 or coarser for flattening repairs or heavy orange peel, and then with great care.

• Typical progression: P600, P800, P1200, P1500, P2000, P2500. For a high gloss, finish at P3000 before polishing.
• Do not skip more than one grit step. Skipping leaves deeper scratches that later stages cannot remove.
• Sand in alternate directions between grits. This helps you see when the previous scratches are gone.
• Soak wet-and-dry paper in water for ten minutes before use. It becomes softer and less likely to clog.

Wet sanding without making a mess

Wet sanding is the safest way to control dust and heat on composites. The water carries away debris and lubricates the abrasive. Use a firm rubber sanding block or a soft interface pad. Fingers alone create pressure points that can cut through gelcoat in an instant.

• Spray the surface frequently. You want a thin, slippery film of water, not a dry patch.
• Use light pressure. Let the grit do the work. If you feel the paper grabbing, add water or switch to a finer grit.
• Wipe the slurry away often and rinse the paper. Trapped particles scratch deeper than the grit itself.
• Stay away from edges and corners. Tape them off if you can. Use a sanding block with a rounded edge for tight areas.
• Check your progress constantly. If you see a different colour or a grey shadow, stop. You may be close to the laminate.

Polishing without burning through

Polishing removes the fine scratches left by P2500 or P3000. It also generates heat, often more than sanding because the pad is in constant contact. Use a dual-action polisher if you have one; it is far more forgiving than a rotary. If you use a rotary, keep the speed low and the pad moving.

• Start with a foam cutting pad and a medium compound. Work a small area, about 300 mm square, at a time.
• Run the polisher at 600–1000 rpm for rotary, or low-to-medium speed on a dual-action. Never press hard.
• Keep the pad flat. Tipping it on edge concentrates heat and cuts through gelcoat fast.
• Check the surface temperature with your hand every few seconds. If it feels warm, stop and let it cool.
• Switch to a finer foam pad and finishing polish for the final gloss. Use separate pads for cutting and finishing.
• Clean residue with a fresh microfibre cloth. Do not let dried compound build up and get dragged around.

Knowing when to stop

The best finishers know that less is more. If you see the weave texture printing through, a grey or dark patch, or a sudden loss of gloss, you have likely gone through the gelcoat. Stop immediately. Continuing will only make the repair bigger. It is far better to leave a tiny defect than to expose structural fibres.

Test your technique on an offcut or a hidden area first. Work in good light, take breaks, and keep your grits and pads clean. With patience and a healthy respect for heat, you can achieve a professional finish that lasts, without risking the composite beneath.