November 27, 2016
by Mike Elliott, Kettle River Canoes
A little research into traditional wood finishing methods shows that, for over a hundred years, there were three basic steps to finishing the interior of the canoe – Oil, Shellac, Varnish. That said, I get a lot of e-mails and comments asking me about this. It appears that much of the knowledge about finishing wood for outdoor use has been lost over the years or clouded by conflicting information.
Note: Oil, Shellac and Varnish are applied to bare wood. If you are not stripping the old finish and simply want to add a coat of varnish to the existing interior finish, start by cleaning the varnished surface with TSP (tri-sodium phosphate) mixed in water. Rinse the interior with clean water and let it dry. Then, use fine steel wool to scratch the surface of the old varnish and make sure that all dust and debris is removed. With the varnished wood properly prepared, it is ready for the application of new varnish as described below.
MYTH #1: Applying linseed oil to bare wood will hamper the adhesion of other finishes. Linseed Oil is the basis of all interior finishing in canoes. I must add that I am referring to “double boiled” linseed oil. The name is rather misleading since the oil is not boiled but rather contains a variety of drying agents (japan drier is often used). Raw linseed oil takes years to dry. This is useful when you want a compound to remain flexible for years (i.e. Marine Bedding Compound such as Dolphinite).
A mixture of boiled linseed oil and turpentine – usually in a ratio of two parts oil to one part turpentine has been the mainstay of wood preparation for exterior use for centuries. The mixture soaks into the wood and keeps it supple and strong for decades. It also prevents water from soaking into the wood thereby helping to prevent rot. I apply a coat of the oil/turpentine mix to the entire canoe and let the oil dry for a couple of weeks (again, if you leaving original varnish on the interior of the canoe, apply the oil/turpentine mixture to the hull exterior). The wood in old canoes is very dry and brittle, so lots of oil is required. I apply a single coat of oil and let it dry for at least a week.
MYTH #2: Varnish will not stick to Shellac. Shellac is fundamental to hard finishes on wood. It creates a superb base for varnish and seals the wood in order for the varnish to ‘build’ properly. It is easy to apply, dries in an hour or two, and polishes quickly with extra-fine steel wool. Back in the days when woodworkers made their own varnish, shellac (as well as other gums and resins) was added to heated linseed oil to create the varnish.
Shellac is made from resins exuded by the female Lac beetle in India. The resin is refined and dried in the form of flakes that range in colour from almost clear, through various shades of amber to dark orange (almost brown). The shellac flakes are sold typically in one-pound bags which are then dissolved in denatured alcohol (Ethanol mixed with a little methanol to prevent people from drinking it. Methanol — also known as methyl hydrate can also be used on its own to dissolve the shellac flakes).
The concentration of shellac in the alcohol is referred to as the ‘cut’. I normally buy pre-mixed shellac at the hardware store which is typically a ‘four-pound cut’ – four pounds of shellac flakes dissolved in one gallon of alcohol. This is a rather thick mix. Most woodworkers prefer a two-pound cut. I dilute the pre-mixed shellac with lacquer thinner (a cocktail of volatile organic solvents usually including acetone, toluene, xylene and methyl ethyl keytone) in a 1:1 ratio. Normally, shellac dissolved in alcohol is anhydrous and turns cloudy white when it comes in contact with water – not a good thing for canoes. The addition of lacquer thinner prevents that from happening and gives me a nice two-pound cut to work with. In fact, shellac dissolved in lacquer thinner (primarily acetone) is often called lacquer. If you are mixing your own shellac from flakes, dissolve them in lacquer thinner alone in order to create a coating that will not become cloudy when it contacts water.
Apply shellac with a natural bristle brush. This stuff dries almost immediately, so application is fast and indelicate. Apply lots of shellac to a small area to ensure full coverage with one brush stroke. Shellac is more slopped on than painted on. Once applied, do not go over an area again — one sloppy brush stroke and move over to the next small area. It is important to maintain a wet edge as you move down the length of the canoe, so speed is the key. Allow the shellac to dry for a couple of hours at room temperature. Then use extra-fine steel wool to polish the surface and create small scratches in the shellac. Remove, any dust and debris and you are ready to apply varnish.
Myth #3: Varnish is difficult to apply. Traditionally, varnish is made by dissolving gums or resins (such as shellac, rosin, mastic, amber, copal and damar) in heated oil (such as linseed oil or cotton-seed oil) and thinned with turpentine (distilled pine sap). These days, most commercially manufactured varnishes contain petroleum-based alkyd polymer resins in oil thinned with mineral spirits (petroleum-based solvent). A side note here is that some commercial manufacturers of varnish use the label Varathane® for a mixture of resins dissolved in linseed oil. Meanwhile, a mixture of resins dissolved in cottonseed oil has been called Urethane®.
If varnish is used straight from the can, the high concentration of solids (alkyd resins) makes it almost impossible to apply without ending up with sags, drips, streaks or bubbles in the finish. Consequently, conventional wisdom states that it is very difficult, verging on impossible, to achieve a smooth, even coat of varnish. However, there is a simple solution. Just thin the varnish about 12% with mineral spirits (paint thinner). Some top-quality varnishes come with a higher concentration of solids and therefore require a little more thinning. In any case, once thinned, the solvent allows the varnish to flow more easily which means that it will self-level to create a smooth surface. The solvent also allows the varnish to dry faster thereby eliminating sags in the finish.
Before applying varnish, prepare the surface of the shellac base-coat or previous coat of varnish by scratching the surface with fine steel wool. Too much rubbing will remove the previous coat, so quick and light is the key. The scratches give the varnish something to hold onto. Otherwise, the varnish will dry and then peel off.
Vacuum the surface thoroughly to remove dust and debris. Then, go over the surface quickly with a tack-cloth to remove any remaining dust.
The interior of a wood-canvas canoe is irregular with lots of gaps and uneven surfaces. Use a natural bristle brush to get the varnish into all the little nooks and crannies. I use a 2” (55mm) brush. It is a relatively major investment (currently costing about $48USD) and well worth it when called into service on a regular basis. I used one brush on more than 100 canoes over a period of about eight years. I finally had to retire it because the bristles had worn down to less than half their original length.
Set up your canoe in a well-lit space with good ventilation, away from direct sunlight. Pour about two inches (5 cm) of thinned varnish into a clean, empty one gallon (4 litre) paint can. Load the brush with varnish and rap the brush against the sides of the can to shake off excess varnish. Apply the varnish quickly and vigorously making sure that it gets into all of the corners. Work on a short section of the canoe. Then, look at the surface from an angle with work lights set up at an opposite angle to reveal any areas that were missed. Apply enough varnish to achieve full coverage while at the same spreading it thin enough to avoid drips or small pools. Don’t worry about streaks or bubbles. If the varnish is thinned properly, it will spread evenly and bubbles will disappear in a few minutes. Once you have full coverage, ‘tip’ the surface by touching it lightly and quickly with the brush bristle tips. It is best to tip the surface first across the grain of the wood and then with the grain. However, it is difficult to tip in both directions in the canoe interior, so I usually just tip in one direction following the grain of the ribs. The varnish is both applied and tipped very quickly. Then, move over to the next section of the canoe. Always maintain a ‘wet edge’ as you apply varnish to the full length of the canoe. Work in small sections to make sure that the varnish in that section is still wet when varnish is applied to the next section. That way, the entire surface will be smooth. Once done, go away and leave the canoe in a well-ventilated, dust-free space for 48 hours. I normally apply two coats of shellac and three coats of varnish.
Clean your natural bristle brush in three stages. First, clean it with mineral spirits or turpentine. Then, clean it with lacquer thinner. Finally, clean the brush with a heavy duty cleaner such as Lestoil®.
All of this (and much more) is described in my book – This Old Canoe: How To Restore Your Wood Canvas Canoe.
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