In clean air untreated teak weathers to an attractive ash gray, but where most boats live, the assault of modern-day air pollutants quickly turns bare teak nearly black. Scrubbing tends to leave behind an unattractive mottled look, neither golden nor gray. Most boat owners eventually find themselves unhappy with either look and decide that some treatment is essential.
Paint provides the longest lasting protection but it hides the wood. If we want the natural beauty of the wood to show, we must apply a clear coating. The choices are oil, sealer, or varnish.
Before teak can be given any coating, it must be completely clean. Your expensive teak is literally dissolved by strong cleaners, so always use the mildest cleaner that does the job. A 75/25 mixture of liquid laundry detergent (such as Wisk) and chlorine bleach may be adequate, perhaps boosted with TSP (trisodium phosphate). Apply this mixture with a stiff brush, scrubbing lightly with the grain. Leave it on the wood for several minutes to give the detergent time to suspend the dirt and the bleach time to lighten the wood, then rinse the wood thoroughly, brushing it to clear the grain.
If the teak is still dark or stained when it dries, a cleaner with oxalic acid is required. This is the active ingredient in most single-part teak cleaners. Wet the teak and sprinkle on the cleaner. Spread it evenly with a Scotchbrite or bronze wool pad, then give it a few minutes to work. While the wood is still wet, scrub it with the Scotchbrite pad or bronze wool. (Never, ever, ever use steel wool aboard your boat — it will leave a trail of rust freckles that will be impossible to remove.) Oxalic acid will dull paint and fiberglass and damage anodized aluminum, so wet down surrounding surfaces before you start and keep them free of the cleaner. Rinse the scrubbed wood thoroughly--brushing is required — and let it dry completely.
Two-part teak cleaners are dramatically effective at restoring the color to soiled, stained, and neglected teak, but these formulations contain a strong caustic and an acid — usually hydrochloric — and should only be used when gentler cleaning methods have failed. Wet the wood to be cleaned. The cleaner will dissolve natural bristles, so use a nylon brush to paint part one onto the wet wood. Avoid getting the cleaner onto adjoining surfaces. Remove the dissolved surface by scrubbing the wood with the grain with a stiff brush or a Scotchbrite pad.
Part two is an acid that neutralizes the caustic in part one, and it usually has some additional cleaning properties. Paint a sufficient amount of part two onto the teak to get a uniform color change, then scrub lightly. Flush away all traces of the cleaner and let the wood dry.
Oiling teak on boats is a time-honored tradition. Oil intensifies the colors and grain patterns of wood and gives the wood a rich, warm appearance. Because it simply enhances the inherent beauty of the wood — more like salt than sauce — oiling is arguably the most attractive of all wood finishes, and it restores some of the teak's natural oils and resins. Unfortunately, the benefit of oiling exterior teak is extremely transitory. The sorry truth is that teak will last just as long if you don't oil it — longer really, since repeated between-coat scrubbing wears the wood away. But oiling teak isn't about protecting the wood; it's about recovering and maintaining that golden glow that made us want teak on the boat in the first place.
Teak oils are primarily either linseed oil or tung oil, bolstered by resins to make them more durable. Linseed oil tends to darken the teak, but it is significantly cheaper. Tung oil doesn't darken the wood, and it is more water resistant than linseed oil — a notable advantage for boat use. However, a month or two after application, it may be hard to discern that much difference since both oils carbonize in the sun and turn dark. Proprietary teak oils address this problem with various additives, including pigments, UV filters, and mildew retardants. Some that perform admirably in one climate are reviled in another. If you are going to oil your teak, make your teak oil selection based on the recommendations of other boatowners in your area.
Apply teak oil with a paint brush. Immediately wipe up (with a spirits-dampened cloth) any drips or runs on fiberglass or painted surfaces, or the resins the oil contains will leave dark, nearly-impossible-to-remove stains. Watch out for sneaky runs below the rail.
Oiling requires multiple coats. The wood will initially "drink" the oil, and thinning the first coat about 20% with mineral spirits or turpentine encourages it to penetrate the wood more deeply. By the third coat, oil will begin to stand in some areas. Wipe up excess oil with a cloth. Continue to brush on the oil and wipe away any excess until the wood is saturated. The wood should have a matte finish without any shiny spots.
Another approach to achieving a natural look is the application of a sealer. Durability and ease of application have made some sealers very popular with boat owners.
Sealers don't feed the wood but, as the name suggests, they seal out moisture and dirt, and seal in natural oils and resins. Unfortunately, the oils and resins may already be lost, so the first step in applying a sealer to old teak is to restore the oil content with a thorough application of teak oil. Clean and bleach the wood to a uniform color, then oil it until it refuses to accept more. Now wait at least two weeks to let the resins dry before you apply the sealer.
After two weeks, wash the wood and let it dry completely. Sealers need an oil-free surface to attach to, so wipe the wood heavily with a rag soaked in acetone to remove all oil from the surface. The oil the wood has absorbed will be unaffected by this quick-flashing solvent. Unless the can instructs you otherwise, apply sealer exactly like oil. A throw-away brush is adequate for the job, and brushing technique is not a concern. Wipe away all excess with a cloth. Apply additional coats until the surface shows a uniform matte finish.
Maintain sealed wood by washing it and applying a fresh coat of sealer every two to three months.
Aside from its much-admired appearance, varnish offers genuine protection for the wood. Wood coated with varnish will not dry out and split, will not absorb moisture and rot, is unaffected by dirt and pollution, and will be untouched, thus unstained, by oily or greasy spills.
The absence of pigment in varnish means it does not shield the underlying surface from the sun. Ultraviolet radiation penetrates the coating and carbonizes the oils in the wood, causing the wood to darken beneath the varnish. To minimize this effect, varnish makers add ultraviolet inhibitors — sun screens — to their products. For exterior brightwork, select a quality spar varnish (not urethane varnish) heavily fortified with UV inhibitors. As always, get local recommendations from other boatowners before selecting a specific varnish.
Teak doesn't hold varnish as well as other woods due to its oil content, but a long-lasting coating is possible with the right technique. Books have been written about applying varnish. You may not get that perfect, mirror-like finish on your first try, but as long as the wood is ivory smooth, the weather is warm and dry, and you don't "worry" the varnish with too many brush strokes, you should get admirable results. Plan on applying at least six coats.