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Showing posts with label marine. Show all posts
Showing posts with label marine. Show all posts

Wednesday, October 16, 2019

Reference: "Wood Finishing for Marine Use"

author, clinical consultant, educator

Wood Finishing for Marine Use

Woodworkers and boat builders are, on the whole, a contentious bunch. They argue about all kinds of things: tools, methods, aesthetics, materials. But their favorite topic, the one to which they have turned with unfailing habit for centuries, approaching it with an alchemical reverence that borders on mysticism, is that of wood finishing. Fine finishes – lustrous, highlighting the wood’s grain, inviting the hand to touch – have long been the pinnacle of wood craftsmanship. Some violin makers still preserve, even today, secret varnish formulas that have been passed down through many generations. The long history of secrecy and experimentation in wood finishing has led to its status as the most complex subject in woodworking. This makes sense; there are, after all, hundreds of ways to effectively finish wood. Depending on the intended effect, excellent results can be achieved with milk, crazy glue, the oil from walnuts, and many other surprising products. Faced with the overwhelming diversity of finishing products on the market today, many people working on boats surrender either to the marketing ploys of manufacturers or to the old habit of finishing everything with spar varnish. But wood finishing can be a joy, and if you follow a few simple guidelines, it can be easy as well.

Install Your Wood Properly

Many problems with finishing, and particularly with refinishing, are unrelated to the finish itself but derive instead from chemical reactions between metal fasteners, water, and wood. If you’re installing and finishing new wood, use dry wood (below eighteen per cent moisture content), and use high-quality, corrosion-resistant fasteners (corrosion accounts for about thirty per cent of all marine equipment failures). Pre-drill and countersink all screws before installation. Make sure that the angle made by the underside of the screw heads matches the angle of your countersink: wood screws usually have an eighty-two degree angle, machine screws are shaped to ninety degrees. If you neglect to countersink effectively, a small air pocket is left inside the screw hole. Over time, this will fill with water or ambient moisture – even beneath a thick finish – and will create a streak or stain, particularly in woods such as oak. If you have the joinery skills, avoid fasteners altogether and use, instead, traditional joints glued with epoxy. Such joints are almost always stronger than those made with fasteners.

Sand, Clean, Sand Again (and Again)

Wood must be sanded before finishing. This series of steps frequently takes as much time as shaping and installing the wood. Don’t take shortcuts here, or rush through. Go slow. Watch for imperfections and rough spots that may appear as you move through the wood layers. Unless the piece is very small, sanding should be done by machine. Avoid belt sanders, which are easy to use improperly, resulting in uneven surfaces and excess wood removal. For sheer violence upon the wood, power washing is even worse that belt sanding. The water stream pounds the wood fibers into uneven grooves, leaving the surface clean but rough. A power washer should never be used in the finishing or refinishing of wood; nor should two-part acid solutions, once popular in the cleaning of teak, that act by dissolving the wood. If you must remove old finish, use a methylene chloride or citrus-based stripper: both minimize harm to the wood’s underlying surface. Usually, new finish can be applied right over older, worn layers of film that are cleaned and sanded properly.
Ideally, invest in (or borrow) a random orbit sander, which abrades in swirling, self-erasing patterns. Random orbit sanders create a uniform surface regardless of the tool’s direction. Don’t push hard – this compromises the swirling action. Start with 80 grit, if the wood feels rough to the touch, but if it has already been smoothed by a power planer (this is the case with most store-bought lumber) you can go straight to 120 grit. Wherever you start, the central goal is to keep the wood surface flat. Don’t dig in with the edge of the sander to reach a low spot. Be patient, let the sander bring the wood into uniformity. You know you’ve completed the work of a given grit when the surface no longer changes as you sand. Each grit creates a particular refractory quality in the wood, and with practice you can learn to distinguish this. Before you change to the next highest grit, you must clean the surface of the wood. Compressed air does a wonderful job (if you’re outside), as do tack cloths, but brushes don’t work well. In a pinch, you can dampen a rag with mineral spirits or alcohol and rub the wood down. Rough grit particles left on the surface will be caught up by the sander and will swirl across the wood, leaving permanent marks. Usually, such marks are not visible until after the finish goes on. Most woodworkers start with 120 grit, move to 180, and finish with 220. You can go as fine as 2000 grit if you want, but it’s a question of diminishing returns, and 220 is typically the best place to stop. Some woods, such as teak, are rich in natural oils, and it’s a good to idea to wipe the wood with a solvent such as acetone after the final round of sanding. This dissolves the surface oils and helps with finish adhesion.

Choose the Right Finish 

Some decay-resistant woods such as teak, cedar, and oak can be left unfinished, though finishing enhances their nautical appearance. A few tropical hardwoods, such as cocobolo and lignum vitae, are difficult to finish properly, and should be left bare. If you have a teak deck, leave it bare. Clean the surface regularly with liquid soap, salt water, and pot scrubbers used perpendicular to the grain. Don’t use a stiff or wire brush, or rub with the grain, as this will have the same effect as power washing. (Tough stains on teak can be removed by judicious application of bleach, or oxalic acid solution.)
The advantage of unfinished wood is that you will never have to refinish it; and if installed properly, it will last generations. Most boat owners prefer, however, the grain-enhanced sheen of an oil or film finish. At this point, you have several options.


Quite a number of oils can be used for finishing (mineral, tung, linseed, walnut, soya, lemon). Their appeal lies both in the color they bring to the wood and in their ease of application: wipe on, wait a few minutes, wipe off. Tung oil (especially if polymerized) is the most resilient of the oil finishes, and if applied in several coats – enough to build a thin film – it provides some measure of protection against the elements. So-called Teak oil is a marketing designation that refers to many different oil (or oil and varnish) formulations. Such products vary widely in quality; the best of them provide protection equivalent to tung oil. Many varnishes can be applied over base coats of oil (check the label). This is the best way to enhance the grain of any wood. Oil finishes are easy to restore and re-apply. They are the simplest, least labor-intensive finishes; but they are also least in longevity.

Oil and varnish 

Many woodworkers enjoy so-called Danish oil finishes, which have nothing to do with Denmark. These are oils (tung or linseed) mixed with varnish, a combination that offers the aesthetic advantages of oil but with greater durability than oil provides on its own. You can make your own oil/varnish mixture by mixing equal parts oil, varnish, and turpentine. Apply the mixture with a rag, wait about twenty minutes until it begins to become slightly tacky, then rub off all the remaining finish. Wait twenty four hours and apply another coat. Four or five coats, applied over a week, yield a lovely, soft look. And because the surface is wiped clean with every coat, there’s no problem with dust accumulation, as there is with varnish. As with straight oil, you can also use an oil/varnish finish as a base for a more durable topcoat such as polyurethane. This provides an excellent blend between beauty and longevity.


At one time, varnish was made from a combination of plant-based resins, linseed oil, and turpentine. Today, most varnishes use blends of synthetic resins: polyurethane, phenolic, or alkyd polyester. Polyurethane, also known as urethane, is more durable than the other resins, but is also more likely to crack in marine environments. Modern varnishes, which are essentially plastic, provide excellent moisture resistance. The ratio between oil and resin determines the coating’s hardness, and therefore its resistance to cracking and peeling. Spar varnish, also called long oil varnish, simply has a bit more oil in it. This makes the varnish more flexible, which is useful for nautical equipment (like spars) in changing weather conditions. As environmental concern increases with respect to the volatile solvents in varnish, water-based products are increasingly appearing on the market. Many offer protection equivalent to oil-based varnish (though the finish looks slightly different).
When choosing varnish, look for tung oil and a base of phenolic resin (flexible) or polyurethane resin (hard and durable). Follow the application directions and pay particular attention to minimizing dust. Resist the temptation to stop at one or two coats. Apply several – as many as six or more – and sand lightly with 220 grit between each (some products don’t require this step). Do not apply varnish in direct sunlight, on damp days, or in high humidity.


The unique depth and color of the finish on fine antiques can be achieved with only one product: shellac. Derived from the secretions of a tiny insect – the lac bug, Laccifer lacca – shellac is widely used in the culinary arts (for glazes) but has fallen out of favor in woodworking since the advent of synthetic resins, which are easier to apply. Yet shellac is a wonderful finishing product: highly water resistant (if fresh, of good quality, and applied properly), non-toxic, easy to repair, and with a lovely hue that no other finish can match. For interior woodwork, shellac is unquestionably the most elegant solution. It’s not as resilient as varnish, nor as moisture-resistant. But if you’re a traditionalist, shellac can set your woodwork apart. The learning curve for proper application is somewhat steep, but the rewards are stupendous. It’s best to buy shellac in flakes and apply numerous sprayed or hand-rubbed coats over preliminary coats of linseed or tung oil (let the oil dry for a week). Each coat of shellac must be very thin, so twenty or thirty coats is typical.

Epoxy and Varnish 

After extolling the virtues of shellac, an ancient finish that has fallen almost entirely out of use, it seems only fair to introduce an entirely new and technological approach. Traditionalists and woodworking purists will object to epoxy on philosophical grounds, but epoxy is superior to every other kind of finish in strength, resilience, and moisture-resistance. The wing skins of fighter planes are made from epoxy.
Unlike varnish, which is somewhat permeable, epoxy completely seals the underlying wood. It’s essential, therefore, to use very dry wood (less than fifteen per cent moisture content) and install it properly. Any remaining moisture could show up beneath the film. And if water enters from the outside and becomes trapped, the wood will rot. Epoxy is also expensive. But with these caveats in mind, epoxy is an ideal base finish.
Many boat builders use the West System formulations, which come in two-part kits containing a clear resin and a special coating hardener (number 207). Follow the directions exactly. Don’t mix the contents by eye; use the optional mixing pumps that dispense precise amounts for each component. After applying several coats, finish the job with two or three coats of good quality varnish; ideally, a two-part polyurethane. This will add color depth and, more importantly, will protect against degradation of the epoxy film from UV rays. Use a gloss if possible, to maximize sunlight reflection. The varnish will protect the epoxy, and in turn the epoxy will provide a stable base for the varnish so it doesn’t crack under temperature-related expansion and contraction. These two combined finishes provide substantially more protection than either used on its own.

A Final Word 

Finishes require regular maintenance and replacement. Don’t expect to get more than a few months from an oil finish, and more than a year or two from varnish. Epoxy and varnish combinations will last longer. More coats of any finish means more durability. Maintenance coats applied every year, or every season with oil, prevent substrate damage and extra work. Do the job properly from the start, maintain the finish frequently, and your wood will shine.

A Postscript about Cetol 

You know the look: a russet orange in the wood, warm and distinctive. And very popular. Cetol, a line of wood finishing products made by Sikkens (a subsidiary of Akzo Nobel, a European coatings company), has made significant progress in the North American market over the past several years. The unique appearance of a Cetol finish (from which traditionalists recoil in horror) suggests a unique product,perhaps something new and more effective. But in fact Cetol is similar to regular varnish: its formulation includes various processed oils and alkyd resin. Fungicide and UV inhibitors are added to the mixture, as they are in most high-end varnishes. And Cetol contains no polyurethane, so the finish remains malleable. The particular hue of the finish derives from synthetic iron oxide pigments added to the formulation. These assist with UV screening, but their hue fades over time, and the film itself begins to degrade – as with other varnishes – in about six to eighteen months. Sikkens does not recommend the use of its regular Cetol products (Cetol 1, 23 SRD, etc.) for marine use. These products are for siding, garage doors, log homes, outdoor furniture and the like. A specialized product, called Cetol Marine (with very similar composition, in fact, to the other Cetol products) is slightly more useful in salt water environments. Its durability is consistent with other quality film finishes: the more coats you put on, the longer it will last. Apply three coats per year for optimum protection and longevity.

© 2019. All rights reserved.

Ross Laird

Ross Laird

author, clinical consultant, educator