Gelcoat is a polyester-resin based top coat added to fibreglass construction to seal and add colour. Gelcoat is mixed in the same manner as epoxies--gelcoat plus hardener--and must be applied quickly after mixing as it can set up in some cases in less than 30 minutes. When mixing gelcoat, colour is added like food colouring to cake batter: blue plus yellow make green. Gelcoat colour bottles come with a mixing chart for making basic colours in specific. How much gelcoat and how many coats you apply depends on the application and the intended colour density.
Gelcoat is mixed and then is a viscous liquid with the consistency of thick cold paint. You can brush, roll or spray on gelcoat. Just always keep in mind it is a resin-based material and will set up very quickly and very hard. Always have plenty of acetone nearby to thin the gelcoat on paint brushes, rollers and especially compressed-air powered spray guns. Forgetting for just a few minutes can result in a ruined tool.
One thing to avoid when applying gelcoat is do not treat it as you would epoxy when bonding layers of fibreglass or composite cloth. Don't pour it on and then quickly spread to voids and around the surface, pushing it into the fabric. Treat gelcoat as you would paint: apply evenly, avoid brush and roller strokes and work in a clean dry environment. Dust and insect landing in sticky gelcoat can ruin the continuous smooth finish.
If you are applying a light colour over dark fibreglass, you will need to apply more than one coat for the gelcoat to be opaque. If you have a black filberglass boat and you want white gelcoat, there is the possibily of four or more coat. Gelcoat this thick, however, can easily crack and will not cure properly. Instead, conside sanding down a signification portion of the existing black gelcoat and then applying a sealing primer paint first. This will then reduce the number of coats to two, possibly three.
If you are gelcoating a boat for saltwater use, then apply a minimum of three even coats of gelcoat. This not only protects the fibreglass from salt, it also inhibits the growth of barnacles and other parasites on boat hulls.
Consider after applying the colour layers of gelcoat to add one additional clear layer. This will protect the gelcoat from the extreme UV radiation from sunlight and inhibit colour fading. Eventually the colour will fade but an additional clear coat can be the difference in five years and ten years before noticeable fading on the boat's hull and other surfaces.