Finishing Oily Woods
By Sal Marino
Traditionally, some of the worlds most colorful woods like rosewood, teak, ebony and cocobolo are often used to build musical instruments, decorative boxes, jewelry, accents and trim on furniture. Recently though, many of these woods are being used to build whole pieces or sets of custom furniture. As more are being used by not only professional but amateur woodworkers, many people are running into difficulty when it comes to finishing of these woods.
The main problem lies in the natural oils and resins that are contained within woods like rosewood, teak cocobolo, etc. The oils create two main problems.
1. When oil based finishes like varnish, polyurethane, Danish oil finishes, and others are applied over the wood, the finish sometimes takes a very long time to dry. All of these type of oil based finishes dry by absorbing oxygen. The natural oils and resins contained in exotic woods will slow down the drying time by retarding the absorption of oxygen into the finish. Sometimes, if you happen to get stuck with a very oil piece of wood, the finish may stay tacky for weeks.
2. Adhesion. While other finishes like nitrocellulose lacquers, pre-catalyzed lacquers and water based finishes dry better over oily woods, the oils may prevent these finishes from adhering properly to the raw wood.
Below, I have included a few different types of finishes and finishing techniques that I have had success with, but first, before applying any finish, you must perform the following steps to remove any oils that may be on the surface of the wood.
1. After preparing the wood by usual methods of sanding, clean all sawdust off the surface.
2. Using a rag lightly dampened with a quick evaporating solvent like acetone, lacquer thinner or denatured alcohol, wipe the whole surface down. This gets all the natural oils off the surface of the wood, but you must work quickly to apply your first coat of finish, for if you dont more natural oils will bleed onto the surface.
While many exotic woods are rarely stained, because the natural color of the wood is so appealing, all have to have some type of finish applied to protect against abrasion, moisture, dirt, dust and sunlight. Over the years, I have had the opportunity to try many finishes and finishing techniques over oily woods, and I have had the most success with the following:
1. Shellac Sealer / Natural Resin Varnish Finish.
If you are going to be finishing a piece of furniture that is going to get a lot of use, (like a table) you will want to use some type of topcoat finish that will protect it against abrasion, as well as spills, dirt and dust along with making it easy to maintain. This finish has worked well for me. After wiping down the surface with quick evaporating solvent, (acetone, lacquer thinner) apply two thin coats of shellac. I use 3 lb. cut clear shellac and reduce it 50/50 with denatured alcohol. Apply the two coats by either spraying or brushing with high quality natural or china bristle brush. Let first coat dry about 2 hours before applying second coat. This will seal the surface and prevent any more natural oils in the wood from bleeding back to the top. Let these two coats dry at least 2 days. Lightly scuff sand the shellac with 400 grit paper. And wipe dust off surface. Next, apply 2 to three coats of a natural resin varnish. DO NOT USE A POLYURETHANE OR ANY VARNISH THAT HAS POLYURETHANE IN IT. IT MAY NOT ADHERE TO SHELLAC. I use a varnish manufactured by H Behlen & Bro. This is called Behlens Rock Hard Table Top Varnish. It is a natural resin varnish that contains no poly. Reduce each coat approx. 20 percent with Behlens Rock Hard Reducer. This works out to 4 parts varnish and 1 part reducer. I use a foam brush to apply this varnish, but if you are used to using a bristle brush and get good results, stick with it. Let each coat dry at least 24 hours (longer if you are in a humid area). Scuff sand very lightly with 320 grit paper between coats. After the last coat is applied, if the sheen does not look even, you may apply a few additional coats until you achieve a uniform sheen. This is a gloss varnish, if you wish to obtain a semi-gloss or satin finish, simply wait about 2 weeks for the finish to cure and then rub out with 600 grit paper and rubbing oil or use 0000 steel wool or Scotchbrite or Sunbrite (these are synthetic non-woven abrasive pads that replace steel wool. Purchase the fine type. The light gray color is usually equivalent to 000 or 0000 steel wool. If desired, you may also apply a coat of high quality paste wax after rubbing.
2. Shellac / Wax Finish
On furniture or wooden objects that dont need maximum protection such as a wall clock, dresser or just trim, I have often just used a few coats of shellac and the applied a coat of paste wax over it. After wiping down the surface with quick evaporating solvent, (acetone, lacquer thinner)
apply four thin coats of shellac using the same mixture and process as described in the previous process. Let the four coats of shellac dry at least 3 days, Then sand lightly first with 320 grit paper to remove any dust nibs and smooth out any brush marks. After sanding with 320, use 600 grit to smooth the surface and leave a mellow sheen. Wipe off dust and apply a coat of high quality paste wax such as Briwax or Antique Wax. Apply the wax with a soft lint free cotton cloth, let it haze over, then buff it out with a clean cotton cloth. This technique will yield a very mellow, low luster finish that is beautiful not only to look at but to touch.
3. For A Natural Look- Simply Wax
When I have to finish a decorative wooden object that will not be handled much, therefore needs little protection but also has to look and feel as close as possible to its natural appearance, I simply apply a paste wax only. Here the color of the wax is important. If you are finishing a lighter colored wood such as teak, use a natural or clear colored paste wax so the natural color will not change much. On the other hand, if you are finishing a darker wood, such as rosewood or cocobolo, I suggest you use one of the colored colored waxes, such as Briwax. If you use a light colored wax on dark woods, the wax may build in the pores and make the pores appear light. Dark wax will blend in better with darker woods and even accent the pores. Briwax comes in a number of colors. Along with clear, it is available in Dark Brown, Light Brown, Antique Mahogany (reddish brown good for rosewoods), Golden Oak and other colors.
REMEMBER. ALWAYS TEST FINISHES AND FINISHING TECHNIQUES ON SCRAP BEFORE USING THEM ON YOUR GOOD WORK.