To hold up in harsh marine conditions, boat topside paint has to be durable, abrasion resistant, UV resistant, fade resistant and flexible. Topside paints are typically one-part enamel, one-part-polyurethane or two-part polyurethane. Here are some things to consider when choosing topside paint for your boat.
Ease of application: If you want to paint your boat yourself, it's easy to use a one-part polyurethane top side paint or one-part marine enamel to apply these coatings with a brush, roller or spray gun. One-part boat paints come in finishes ranging from flat to glossy, have good color retention, and adhere well to a wide wide range of marine primers. They dry fairly quickly, and allow you to get glossy, professional-looking results with less effort than using a two-part polyurethane.
UV stability: A UV-stable marine paint not only makes the color fade-resistant, it makes the coating last longer, which helps protect the surface underneath. It's true that two-part polyurethanes are much more UV stable and last longer than one-part polyurethanes, but unless your boat is in a tropical climate, a one-part polyrethane paint will provide UV resistance that is more than adequate - it will also cost less and be easier to apply.
Substrate: Is your boat made of wood? Fiberglass? Aluminum? Enamel boat paint is flexible when cured, so it's a good choice for wooden boats because wood flexes with changes in temperature and humidity. Fiberglass boats benefit from a hard, durable, polyurethane topside paint that won't lift, peel, or crack. Aluminum boats need a copper-free topside paint that won't cause galvanic corrosion.
Finish type: High-gloss and low-sheen finishes each have different advantages. Glossy paint reflects ultraviolet rays instead of absorbing them, so it tends to have greater UV resistance. However, glossy paints also make surface imperfections stand out, including uneven fairing, sags, hairs, and dust. Glossy finishes also become slippery when wet. Low-sheen and no-sheen paints, such as matte, satin, and flat finishes, are better at hiding imperfections, and aren't slick when wet, so they're better for cabin soles, galleys, and other area where wet traction is needed. For maximum traction on decks, docks, and steps, use a ready-mixed, low-sheen non-skid topside paint containing texturing agents.
Abrasion resistance: What kind of wear and tear does your boat get? Does your dinghy or kayak get dragged over rocky beaches? Do you take it on and off of a trailer? In general, one-part paints have some abrasion resistance, but two-part paints are harder when cured, which makes them more abrasion resistant.
Continuous submersion: Many topside paints allow a few hours to a few days of continuous submersion, but most aren't designed be under water continuously for long periods of time. Be sure you're aware of a paint's tolerance for submersion, if that's a consideration for your boat.
Price: One-part marine paints are typically less expensive than two-part paints. To compare, many two-part polyrethanes can cost 2-4 times more per square foot of coverage, than one-part polyurethanes.