Topside paint refers to the finish applied on a boat's upper hull and deck areas and always above the waterline. Topside painting is done above the waterline to protect your boat from the sea, sun, and sand. Prolonged weather and UV exposure deteriorates wood and even fiberglass.
Your boats topsides cover a lot of territory. Working upward from the waterline, the topsides include the boats entire upper hull and deck areas. Technically, the topsides also include the cabin top, the raised portion of the boat above its flush deck.
Most marine paints are designed to withstand weather exposure, plus the effects of direct and reflected ultraviolet light. However, some marine grade finishes differ in their ability to withstand sun and water exposure.
Topside paint must be flexible enough to withstand a boats hull expansion and contraction, especially true of paint used on wooden boats. Any finish chosen needs to resist abrasion and moisture. There are essentially 3 kinds of topside coating: Two part polyurethanes, one part polyurethanes, and enamel paints. Each type of paint has advantages and disadvantages. Deciding which is 'right' for your application depends on your boat's construction and your expectations.
Widely used polyurethane paints are known for their glossy shine that looks almost like factory-applied gelcoat finish.
A one-part paint (single-part polyurethanes and mono-urethanes) gives you a glossy sheen that is harder than enamel. However, the finish is not quite as durable as a professional two-part polyurethane finish. One-part paints are known for their easy application via brush, roller, or spray. Many DIY boaters choose 1-part polyurethane paint to improve the appearance of their boats topsides.
Two part polyurethanes yield the most durable, scratch resistant finish. The hardness means it is not flexible, and will crack if not applied to rigid substrates like fiberglass. Polyurethanes produce a high gloss that most closely resembles a factory gelcoat finish. Popular finish coat over tired, crazed fiberglass gelcoat that has first been properly primed with 2-part epoxy primer. Polyurethanes produce a much thinner, almost watery coating compared to enamels, thus requiring several applications. Best finishes are achieved by roller tipping and spraying. Mix ratios are critical, and produce more wasted paint from unused catalyzed discards. This makes the cost of materials higher.
But only two part polyurethane topsides yield the highest gloss and most durable finish that may hold up as much as five years. Scratch and ding repairs to polyurethanes are visible. Make sure to only apply in well ventilated areas and wear a vapor respirator.
Marine-grade enamel paints are the best choice for traditional wooden boats. Wood regularly swells and contracts, and enamel paints soft coating has the same flexibility.
However, softer enamels are not as durable as harder polyurethane paints, and you must apply a yearly maintenance coat. On the positive side, enamel paint makes the scratch-and-ding touch-up process much easier. Plus, enamel paints are generally self-leveling for easy application, as long as you choose a quality brush. Inert fumes make them more suitable for indoor projects.
Applying topside paint requires meticulous attention to detail, and rushing the process will produce substandard results. For the best outcome, have all your supplies handy, and methodically apply the paint using one of these two techniques.
Rolling and tipping is a non-stop, two-person operation that relies on careful coordination and controlled speed. To begin the process, ensure that you have enough properly thinned paint in your paint pot and roller tray to finish each large section in one pass. In addition, keep more thinner handy so you can replace the thinner that evaporates.
Next, smoothly roll on a coat of thin topside paint. Before the paint can begin drying, your painting buddy will use light brush strokes to remove the just-applied paints roller stipple (the orange peel-like texture). This action also smooths out the surface.
You have three options for rolling and tipping directions: roll horizontally and tip horizontally, roll vertically and tip fore and aft, or roll fore and aft and tip vertically. Choose one method, and stick with it, or you will risk a confused-looking result. Remember, keep the roller and brush moving so the paint does not dry and cause the brush to drag along the surface.
Done correctly, you will see high-gloss results that approach a sprayed finish coat. Note that you can also apply the topside primer with the roll and tip method.
For a project this important, take time for a dry run before tackling your boats hull. Grab your painting partner, and practice your painting techniques on some primer-coated plywood.
Before you begin spraying boat topside paint, know that 1-part and 2-part polyurethane paints have different chemical compositions. Specifically, 2-part polyurethanes may contain highly toxic isocyanates.
Spraying 2-part polyurethane paints without supplied-air respiratory protection, along with special air handling/filters and masking materials, can be extremely harmful to your health. For that reason, we recommend you let professionals handle this potentially hazardous operation.
With proper protection, you can spray 1-part polyurethane paint on your boats topsides. Before you begin, pull on a hooded paint suit, vapor respirator with the correct cartridges (not a plain dust mask), goggles, and gloves. In addition, confirm that you have the proper spray tip for your spray gun and the specific topside paint.
To spray the paint properly, perform three actions at once:
Aiming the spray pattern: Hold the gun upright at a consistent distance from the surface while spraying. Move your arm while keeping the gun pointed straight at the target surface for a smooth, even finish. If, instead, you keep your arm still and move the gun in a fan pattern from side to side, the finish will be uneven.
Pressing and releasing the spray trigger: Start moving the gun before you press the trigger. Continue your arm motion after releasing the trigger to avoid creating added thickness at the beginning and end of each stroke.
Proper overlapping technique: To apply the paint evenly, each time you make a pass to spray, overlap by 50%. Point the spray tip at the bottom edge of the previous spray pass to ensure efficient overlap.
We generally do not recommend it, as blistering and bubbling are a problem for boats left in the water. Interlux notes that most topside painted surfaces will blister when kept wet or constantly immersed. If the boat only remains in the water for a few days, however, a topside coating can be used below the waterline.
Although 2-part polyurethane paint is the most glossy and most durable marine paint for fiberglass boats, its not a DIY-friendly coating. For DIY boaters, 1-part polyurethane paints like TotalBoat Wet Edge, Interlux Brightside, and Pettit Easypoxy are an affordable, easier-to-use option. You can get high gloss, smooth, durable finishes via the roll and tip method. Compare this technique with the expensive, complex systems required when using 2-part polyurethanes.
The short answer is yes, you should prime your fiberglass boats hull before painting it with marine deck paint. The primer helps the paint to better adhere to the fiberglass, and will also hide slight imperfections. Many primers are also sandable, a useful quality when you are worried about finish coat adhesion.
Most importantly, use the paint manufacturers recommended primer and finish coat application techniques. Think of the primer application as a warm-up for the finish coat process. When that final step rolls around, you will be confident in your rolling and tipping skills.