Not all gelcoat products are created equally, so it's important to start with one that is designed for brush/roller application. Many gelcoat products are designed for spray application, and will lack the self-leveling properties that brush application requires. If the gelcoat does not contain wax, paraffin waxes are available to add to the gelcoat as an air-dry additive. For best results, take extra care to use only the amount of MEKP catalyst that is needed for the amount of gelcoat and the environmental conditions.
Do not attempt to apply gelcoat when the conditions are not right, and always avoid applying in direct sunlight. This will give the gelcoat time to flow and self level. It should not be necessary to thin the gelcoat. When brushing, always use a natural bristle brush that can tolerate the gelcoat, while also choosing one that has tapered bristles that are not too stiff. When rolling, always use a 1/8" or 1/4" solvent-safe roller cover that can tolerate the solvents of the gelcoat, as well as one that will not leave lint or hairs in the gelcoat.