Most modern paints are parts of what manufacturers like to call "protective coating systems," which include compatible fillers, putties, fairing compounds, thinners, and undercoats.If you're planning anything more than a touch-up, ask for the full manufacturer's directions because, all too often, one kind of paint will not stick to another's undercoat, even if it's made by the same manufacturer.Marine paints can be divided into two groups: topside paints and bottom paints.There are at least four major kinds of topside paint used on boats, each with its own characteristics and applications:Alkyd enamel. Marine alkyd enamels are nothing more than formulations of oil-based exterior house paint. On boats, this type of paint is great for interior spaces and makes an inexpensive deck paint. Thirty years ago, alkyd enamel was also the leading choice for topside paint, and it still does the job. But for a fiberglass hull, which-unlike a wooden hull with its constant working-provides a stable platform for a long-lasting paint, polyurethane provides a longer life and a higher gloss.Single-part polyurethane. This is a urethane-modified alkyd enamel, a compromise between alkyd enamel and true polyurethane, which offers good gloss, three- to five-season longevity, and moderate ease of application.Two-part linear polyurethane (LPU). This is the best choice for high gloss and a long life, but it's a problem to apply. A professional polyurethane job is sprayed-not recommended for the amateur even if wearing the requisite respirator. Several brands are formulated for application with roller and brush; done correctly, the result is almost indistinguishable from a spray coating. Ideally, it's a two-person job: one to roll on the paint with vertical strokes, the other following behind with a paintbrush to tip the paint with horizontal strokes. As always with marine coatings, preparation is the key to success. The primer will be proprietary to the topcoat.Epoxy. Epoxy is sometimes used as a primer over porous or crazed gelcoat under a topcoat of marine alkyd enamel, but its high susceptibility to ultraviolet degradation renders it unsuitable as a topcoat.Bottom paints are mostly antifouling paints; that is, they contain a compound to deter or prevent the growth of marine organisms, such as weed and barnacles, that would slow the progress of a boat or damage the hull surface. There are at least four common types of bottom paint: ablative, sloughing, modified epoxy, and vinyl, all of which usually contain a copper biocide. Teflon paint has no biocidal properties, and is used on racing craft removed from the water after use. Bottom paints may be incompatible, too, so you must find out what type of paint is on your boat's bottom before you paint over it