What Is Gelcoat and How Is It Applied?

Gelcoat is the generic name for marine exterior epoxy-based boat paint. Boat exteriors are usually completely painted with a thick layer of gel coat and bottom paint is then painted on top of that, for protection against marine growth. Gelcoat is very durable, usually lasting around thirty years but also very expensive, currently around $150 – $300/gallon, with additives.
Painting with gelcoat is straight-forward but there are lots of ways to make a mess of it and it requires a week for testing and various drying times. It’s a completely more scientific process than regular painting but one that can be mastered by a beginner with patience, the first time. Always use the same brand and type of gelcoat and ALL chemicals, as the chemicals from one are only weakly compatible with those of another. Measurements must be precise. If they are not, the gel coat will not be even and won’t match or be repeatable. Once the formulations are proven, write them in the back of the owners manual or somewhere they can be found and gratefully used next time!
This is a very brief overview of the process.
(1) Make, fair and clean the underlying, rigid surface with a tack cloth.
(2) Wipe it with an evaporative degreaser (such as acetone) on a lint-free cloth.
(3) If the surface is subject to immersion, appropriate barrier chemicals must be applied at this stage.
(4) Paint the area with surface prep liquid.
(5) Mix the epoxy primer. For relatively small jobs, I use disposable syringes and cut the tips very short, allowing me to transfer the liquids quickly. I lay them aside neatly in order, allowing for reuse.
(6) Apply the epoxy primer. If spraying, add up to 40% thinner.
(7) Mix epoxy paint in the following order: Resin, hardener, thinner, pigment, flattening agent.
(8) When barely dry to touch but tacky when pressed, apply paint, again mixing thinner – 10% for brush and up to 40% for spray.
(9) Apply UV-protective layer, as specified by paint manufacturer – This is an important step for the life of the repair and will deter UV deterioration, which otherwise will be noticeable.
(10) Wax.
Build up color first and then add flattening agent on the last pass – It’s clear, so it’s almost impossible to build the color with the flattening agent in every coat. it’s vital to use a large spare board and make and apply a number of mixes because the curing process changes color significantly, only gaining its lasting color about five days after application. Generally, use much less pigment than you would imagine. When matching old gelcoat, the last coat will be permeated with flattening agent – Read the instructions for the proper mix. Wet sand between coats, with 600 grit paper and final coat with 2,000 grit paper on a board: Do not use a palm when sanding the final coats because you want the surface to be flat with the surrounding paint. Another tricky issue is the thickness of the gelcoat and you have to guess about the amount of thinner, flattening agent and pigments to build the right thickness. Chipping a bit of loose, existing gelcoat off is a handy tool for comparison.