A stroll through any marina will reveal that the apparent response of many boat owners to this eventuality is little more than a shrug. And why not? Boats sail just as well sans gloss. But the pleasures of a sailboat are not limited to a cracking reach or a smooth tack, and as the surface of the gelcoat erodes, it can take with it a good portion of the joy of boat ownership. If you don't enjoy your boat as much as you once did, maybe it's because it no longer evokes the same sense of pride. This typically has less to do with how a boat sails than how it looks.
Is it really possible to get a great looking finish from paint applied by the "roll-and-tip" method? That is the question I am asked more often than any other, and the answer is an unequivocal yes.
What they were seeing was not superior skill but higher gloss. Spraying polyurethane requires an aggressive (and dangerous) solvent that flashes off quickly, taking gloss with it. The slower solvent for roller application doesn't have this effect. These sailors saw only the gloss; none noticed that the paint had been applied by a roller.
There is nothing particularly difficult about applying polyurethane to the uninterrupted expanses of hull topsides, but getting perfect results takes a little practice. You can get that productively by painting your dinghy first. If you don't have a hard dinghy, a few inquiries will usually turn up one that the owner would be happy to see refinished. The worse condition it is in, the better. If you can't, scrounge up a dinghy, paint a dock box, a discarded hatch cover, or even plastic laminate (Formica).
Start by washing and dewaxing the fiberglass. Even if the hull (or, in this case, the dinghy) has never been waxed, the pores in the gelcoat are still harboring mold release that will interfere with the adhesion of the paint. Use a dewaxing solvent and turn and change your rag often.
Sand the dewaxed surface lightly with 120-grit paper—a palm sander makes this go quickly—then fill any obvious gouges with epoxy filler. Sand again and apply two coats of epoxy primer, sanding between coats. The primer will normally take care of both crazing and porosity; use fairing compound between primer coats to fill larger flaws. When you have sanded (and wiped) the second primer coat to a flawless surface, you are ready to apply the polyurethane.
Start with slightly less thinner than the manufacturer recommends, then brush a small amount of this mix onto the vertical glass. If the paint runs or sags, you already have too much thinner, but it is more likely that you will have too little thinner at this point, exhibited by brush marks in the paint. If these do not disappear entirely in a couple of minutes, add a little thinner and try again. Don't get impatient; keep thinning and testing until the stroke marks disappear.
Pour this mixture into your paint tray and use your foam roller for one final test application on the glass. Lightly drag a dry brush through the paint to smooth it and eliminate bubbles. You need a top-quality badger or an ox-hairbrush for this. Wait a minute or two and the paint should flow out mirror-smooth.
Now just do the same thing to the surface of the dinghy, always working a single wet edge and dragging your brush out through the new roller application from just behind the old wet edge. The job is easier with two people, one rolling and one tipping. Painting with polyurethane is a sprint. Work fast and never go back. You will sand out any flaws after the paint dries.
Mix and test your paint again to get it exactly right, then apply the second coat. Two coats are normally adequate, but if you aren't satisfied, apply one more. By then you will have the hang of it, and your results are sure to be spectacular. You are ready now to put the same mirror finish on your hull.
The new shine will last into the backside of the next decade; so, too, will the satisfaction.
Six Steps Toward a New Gloss
3. Carefully following directions, mix the polyurethane paint. Do a propped-up-window-glass test. Then pour into tray and repeat propped glass test.
4. Employing the roll and tip method, apply the paint, using a foam roller, and follow up with brush.
5. Allow coat to cure overnight then wet sand with 340-grit paper.6. Wash off sanding scum. After mixing and test paint again, apply second coat.
Old Hull, New Gelcoat by Tom Wood
Recovering the Shine by Don Casey
Blisterama Battle Royale—Surviving the Haulout by Mark Matthews
Buying Guide: Battery Switches