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Painting with Marine Polyurethanes
Painting a boat used to be so easy. All you needed was a can of exterior enamel from the hardware store, a clean brush, some masking tape, and a clear day. A quick-and-dirty 'workboat finish' is still an option worth examining if you are out of time and patience.
Today, boatbuilders have access to an arsenal of high tech coatings that can make a boat gleam like a new car. Two-part marine polyurethanes, such as Interlux Interthane, have a deep, rich gloss and abrasion resistance that exceeds fiberglass gelcoat. One-part polyurethanes are almost as glossy and durable, but far less expensive and much easier to apply. Old fashioned marine enamels and even water-based marine paints are also available, but overall most boatbuilders feel that one-part polyurethanes, such as Interlux Brightsides, offer the best finish for the effort required and for the money spent.
Prepare the surface - Unfortunately, paint doesn't always stick to the modern, high-tech epoxy that we apply to our kayaks. The problem lies with a waxy surface residue known as 'amine blush' that's created on some epoxies as a byproduct of the epoxy cure. The MAS and System Three epoxies that we sell don't blush. But some other epoxies do blush. And if you don't deal with the amine blush, your laboriously applied coat of paint will never dry; instead it'll turn into pigmented bubble gum that must be scraped off with a razorblade.
Remove the amine film and other contaminants - After your hull is sanded, wipe down the hull with denatured alcohol on a sterile rag. (Rags with fabric softener or residue from your car's dipstick are big trouble.) Avoid using acetone; it's alleged to have contaminants that leave paint-killing residues.
The Interlux folks recommend further isolating the epoxy from the paint with an epoxy barrier coat. We've never used such a coating. Ostensibly it bonds well to an epoxy surface and seals away all of the associated contaminants while providing a hard, paint-ready surface.
What About Ordinary Primers? - An ordinary coat of oil-based primer will enhance the appearance of your hull by filling low spots that you can neither see nor feel until it's accentuated by the intense gloss of the polyurethane paint. A ‘hi-build' primer, such as Interlux's Pre-Kote Primer will even conceal some of the epoxy runs and other warts that you didn't catch before running out of sandpaper. Caution: ‘hi-build' primers contain titanium dioxide, or talcum powder, and once cured can absorb enough moisture to defeat subsequent layers of paint. To avoid moisture build-up, apply it only on dry days, and apply your finish coats as soon as the primer is cured and sanded. Sand the primer thoroughly with 220 grit paper. Sanding hi-build primer sends up huge billowing clouds of white dust, so it's best to do it outside on a windy day. Fortunately, primer sands fast and leaves a very smooth surface. Don't worry if you sand through the primer where there are high spots; this is normal.
Use A Foam Roller And Foam Brush - Marine polyurethane paints must be applied thinly and evenly or they will 'sag.' Foam rollers and brushes are perfect because they don't hold much paint and you can throw them out when you're done. Use the roller to spread the paint; you should apply the paint so thinly that the roller almost feels dry. Depending on how quickly the paint is tacking up, stop every 24 inches or so and 'tip' out the bubbles left by the roller with a foam brush. Use only the lightest pressure on the brush, and always maintain a 'wet edge': make brush strokes in one direction only, moving from dry surface to wet. Never go back with your brush to catch 'holidays' or sags; these paints tack up very quickly, and you will invaribly make the ugly spot worse. Save the fix for the next coat.
How Many Coats Of Paint? Applying the paint so thinly will probably require three coats for coverage, depending on the color and your patience. Wetsand with 400 grit between coats for maximum smoothness and to avoid removing too much of the thin coat.
Painting with polyurethanes, and its complicated preparations, can consume a large percent of your total building time. We think, however, that mirror-smooth hull paint of a tasteful color, set against a varnished deck and trim, is as perfect a boat finish as one can get.