Know how: Varnishing your Sailboat
There are few boatowners who have never walked the docks and marveled at a beautifully varnished rail or hatch cover. Even boats with no exterior varnish often have some brightwork down below; fiddles, door frames, bookshelves, and mast supports are often finished bright to break up a stark white interior. Any brightwork on your boat, inside or out, needs regular maintenance to stay in top condition.
Varnishes are expected to fulfill two important functions—they enhance the natural beauty of the wood and protect it from the elements. One of the greatest threats to varnish is sunlight. Though all exterior varnishes should contain UV inhibitors, all finishes will eventually break down when continuously exposed to bright sun. As the sun penetrates the finish, it lifts the varnish from the wood. You’ve probably seen a teak toerail whose wood is turning black under a smooth exterior. Because the varnish is no longer adhering to the wood, it must be removed and replaced to restore the finish and provide the necessary protection.
The greatest investment in varnishing is the time put into applying it, so a cheap $20 varnish may not be such a bargain. It’s worthwhile spending extra for a top-quality varnish and having the finish last as long as possible. Be sure to use a varnish that is designed for marine use.
ONE-PART VARNISH. There are three basic finishes for wood: one-part varnish, two-part varnish, and oil finishes. Within these categories are a number of variations—clear, satin, gloss, and matte finishes; polyurethane and tung oil bases; products that do and don’t require sanding between coats.
One-part varnishes can be used straight from the can. Their three main components are an oil, a solvent, and a resin. These varnishes are somewhat flexible and can expand and contract as weather affects the wood they protect. This makes them an ideal choice for covering large areas on boats.
TWO-PART VARNISH. These finishes come in two parts—a clear coating and a hardener or catalyst. When they’re mixed together in the correct ratio, a chemical reaction occurs that results in a very hard, tough finish. Once mixed, the application process for a two-part varnish is the same as for a one-part varnish.
OIL FINISHES. An oil finish has a higher oil content than a varnish and less solvent and resin. One of the great virtues of an oil finish is speed of application, even if numerous coats are used. But oil finishes must be applied more frequently than varnish. I’ve had good results using an oil finish belowdecks, where it holds up well.
Suppose you’ve just bought a used boat. How can you tell what surface coating has been used and whether a new coating will be compatible?
One way to check whether the surface treatment will be compatible is to tape off a small, inconspicuous area and try applying a thin coat of the desired finish. Leave it for 24 hours and if it dries with no signs of crazing or alligatoring then the two should be okay together. It is worth noting that two-part varnishes are hard and can be over coated with a one-part finish but the reverse is not true. A two-part product used over a one-part finish will crack and flake due to the movement of the flexible undercoats.
The one sure way to solve compatibility problems is to remove all the old finish and start fresh. This sounds like a lot of work, and it can be—especially if you have acres of varnished wood aboard. But it’s the best way to revive an existing surface in poor shape, and it could save you time (and money) in the long run.
Over coating an oil finish with varnish is often less of a problem, as oils are often present in the composition of the varnish. Even so, you should try the two together on a small test area.
One thing I’ve learned over and over is that the best finish starts with meticulous preparation. Stating off with bare wood is in many ways easiest, because you don’t have to worry about compatibility with any previous coatings. Especially if the old coating is in a state of disrepair, it’s best to remove it completely. I like to use a hot-air gun and a sharp scraper. Use sandpaper only as a last resort; it’s slow and expensive, it gums up, and it creates a lot of dust.
If the previously varnished surface is in good condition, you can give it a thorough wash-down with plenty of clean water and then sand the surface with either 320- or 400-grit wet-and-dry paper. Use it wet to smooth and prep it for additional coats of varnish.
Using the correct application technique is the key to a perfect finish, which is why it’s imperative to follow the instructions on the varnish can to the letter. Don’t varnish on a damp day or when the air is full of dust. It pays to use the best brushes you can lay your hands on; natural bristles are best. Keep the brushes you use for varnishing separate from paint brushes and use them only for varnishing.
After many years of varnishing, my personal preference is to have a rubbed oil finish belowdecks and a gloss-finish one-part varnish on all above decks brightwork. I know many varnishers who swear by their own favorite brand, and they get nice results.
I strongly recommend that you choose one finish and then stick with it. Get to know one product well, learn how to apply it properly, and you’ll be rewarded with a finish your boating neighbors will admire.
There’s a lot of advice on varnishing available. My favorite is Brightwork: The Art of Finishing Wood, by Rebecca Whitman (International Marine, 1990). Lavishly illustrated with stunning photography, it contains a wealth of knowledge on all things that deal with varnishing.
Sanding off an existing finish is often the least effective method; it makes clouds of dust and is hard work. Use a hot-air gun and a sharp scraper; the finish will come of much faster and with a lot less damage to the underlying surface.
PROS AND CONS
PROS: Less expensive than two-part varnish; flexible; easy to apply
CONS: Not as durable as two-part varnish; must be applied in warm conditions
PROS: Very hard surface; ideal over an epoxy finish
CONS: Expensive; can be tricky to apply properly
PROS: Easy to apply; economical
CONS: Can look artificial especially if a stain is added; needs to be redone more often than varnish