How do you apply gelcoat to repair a boat hull?
I have a few deep scratches and a couple of chipped areas on my hull, mostly around the waterline and the bow. I want to repair them properly with gelcoat rather than just filling them, but I've never worked with it before. How do I prep the area, mix the gelcoat, and get a smooth finish that matches?
Answer
To gelcoat a boat repair, you must grind out the damage to solid fibreglass, apply a catalysed gelcoat mix, and seal it from the air to cure. Once hardened, the repair is wet-sanded through progressive grits and compounded to match the surrounding hull finish.
Preparing the Damaged Hull
Wear an organic vapour respirator and goggles. Styrene fumes and fibreglass dust present severe health risks. Remove all loose material around the chip or scratch. Use a Dremel tool fitted with a tungsten carbide burr bit to rout the damage down to clean solid laminate. Create a shallow V-shape with bevelled edges. Gelcoat fails to adhere to a sharp vertical drop.
Blow away the dust and wipe the area with a clean rag soaked in acetone. This removes surface waxes and contaminants. Apply a border of masking tape close to the edge of the repair to protect the surrounding hull from accidental scratches and overspill.
Matching Colour and Mixing Resin
Standard marine gelcoat is an isophthalic polyester resin. Standard gelcoat cures tacky when exposed to air so subsequent layers of fibreglass can bond to it. For a surface repair, you must use gelcoat with a wax additive (often sold as flowcoat or gelcoat paste by brands like Scott Bader or CFS) or add a styrene wax solution yourself. The wax rises to the surface, sealing out oxygen and allowing the resin to cure hard.
Matching an aged hull colour takes patience. Buy a neutral or white gelcoat and a set of marine pigment pastes. Add the pigment in tiny increments on the end of a mixing stick before adding the hardener. Smear a test dab on the hull to check the match. Gelcoat usually darkens slightly as it cures.
Add the MEKP (Methyl Ethyl Ketone Peroxide) catalyst. The standard ratio is 2% by volume at an ambient temperature of 15 to 20 degrees Celsius. For a small 20ml batch in a mixing cup, this equates to roughly 8 to 10 drops of MEKP. Mix the batch aggressively for at least one minute.
Applying the Gelcoat
You have about 10 to 15 minutes of working time before the gelcoat starts to kick. Use a wooden mixing stick, a plastic spreader or a disposable brush to work the gelcoat into the gouge. Press the resin firmly into the bottom of the V-groove to expel trapped air bubbles.