Presentation by Lynn Shoger

Transcription

1 Presentation by Lynn Shoger Finishing for us woodturners can seem complicated and confusing, especially when we are confronted by a seemingly endless selection of products offered by crafts suppliers and our local Home Centers. In preparing for this presentation I turned to a respected woodworker and writer on the subject, Bob Flexnor, and in particular, one of his books on the subject, F/exnor on Finishing. Flexnor is a woodworker with over 25 years experience as a professional woodworker, 18 years of which he has spent studying, writing about and teaching all aspects of finishing wood. This presentation will provide an introduction to the range of clear wood finishing options available to you, the woodturner, today, in the following categories: Oil Shellac Varnish Lacquer Wax Stains, dyes and opaque finishes (i.e., paints) will be covered at another time.

2 Oil finishes are, undoubtedly, the oldest known finishes. Natural oil finishes, which come from plant seeds and nuts, include linseed oil from flax seed, tung oil from tung nuts and walnut oil from walnuts. Another oil used in finishing is mineral oil which is a petroleum based product. Oil finishes, while providing moisture protection to the finished wood surface, are not very durable when used in their natural state. These finishes are the simplest to apply, but, they are the least durable of finishes. Their appearance after application will eventually become dull. The appearance can be restored with another application of the oil. Mineral oil is a non-drying oil which means that when it is exposed to oxygen (air) it does not undergo a chemical change (i.e., curing) which causes it to harden. Mineral oil is a non-durable finish since the oil will eventually evaporate. Walnut and soybean oils are semi-drying oils which will only dry (i.e., cure or harden) when exposed to oxygen (air) after many weeks. These are not recommended for use as a finish in their pure state. Linseed oil and tung oil are drying oils and will cure (i.e., harden) within a few weeks. With the addition of dryers (catalysts that speed the introduction of oxygen) curing can be accelerated to a day or so. The process of applying oils as a finish involves wiping on a coat of the oil, allowing it to set for a short period and, then, wiping off the residual oil that has not soaked into the wood. This process can be repeated in several times to saturate the wood. However, if, after wiping off any excess oil, oil reappears, "bleeding" onto the surface, this oil should be wiped off until the bleeding stops. If residual or bleeding oil is not removed the surface will become "gummy" from the uncured oil. Oil finishes can be thinned and cleaned-up using mineral spirits. While oil finishes likely date to many thousands of years in the past, the use of shellac is relatively recent to those of us in the western world dating to the 18th or the 19th century. Shellac is made from a secretion from the lac beetle found in Southeast Asia. This durable finish was once highly sought after by woodworkers.

3 The secretion from the beetle is purified and dried into flakes. Shellac in flake form has a relatively long life. In order to create a finish the flakes must be dissolved in denatured alcohol. However, once the flakes are dissolved to prepare the finish, the clock starts ticking on its useful life. The finish is best if used within a year to a yearand-a-half. After that the finish will begin to not cure completely resulting in a finish that is soft and ultimately gummy. If you choose to use this finish you have two choices. You can purchase shellac finish in quantities that will be used within the expected shelflife, or, you can purchase the flakes and denatured alcohol and mix the finish as needed. Apart from finishing, shellac is useful sealing off problems in wood. This is especially true when refinishing a piece of furniture. These problems include: "fisheye" caused by silicone oil in polishes, poor bonding from residual wax from strippers and odors from smoke or animal urine. About the only problem encountered when finishing new wood is resin from pine knots and oily resin from exotic woods. Mixing your own shellac is easy and straight-forward. The standard mixture is a "two pound cut". For our purposes a "two pound cut" can be made by adding a pint of denatured alcohol to one-quarter pound of shellac flakes in a glass jar. Stir the mixture until all of the flakes dissolve. Another shellac finish that may be useful is 'friction polish'. You can purchase this finish commercially, or, make your own by adding an oil such as boiled linseed oil to your shellac mix. Using a commercial shellac, such as Zinser Bullseye, mix 1 part shellac (by volume) with 0.4 parts denatured alcohol and 0.7 parts boiled linseed oil. Apply to your work while on the lathe using a pad of folded paper towel with a liberal amount or polish. Apply the finish evenly, then, increase pressure on the turning until the finish is dry. Long considered the workhorse of woodworking finishes shellac lost favor in the late 1960's and early 1990's with the arrival of wiping varnish. Advantages of shellac: More water and scratch resistant than oil and oil/varnish blends Better dust-free results than varnish or polyurethane which cure more slowly Less polluting and health hazard than varnish, polyurethane or lacquer Easier to apply and richer looking than water-based finishes Easier to clean-up than oil, varnish, polyurethane and lacquer Disadvantages: Not water or scratch resistant when used in work areas Available only in gloss sheen Tends to ridge at edges of brush strokes Slowly deteriorates after being dissolved in alcohol

4 Varnishes are the work horses of the finishing world. They are created by cooking an oil (usually soybean oil, or, occasionally linseed oil) with a resin (usually an alkyd or polyurethane). The cooking causes a chemical reaction which results in the formation of a new material. There are three common types of varnish. Polyurethane varnish is the most protective and durable. Spar varnish is more "flexible" (meaning it better accepts expansion and contraction in wood) and contains a higher ratio of oil to resin. Alkyd varnishes are found across the spectrum of varnishes and are used in oil paints. Varnish has a couple of characteristics that can make it difficult to work with; it has a thick consistency and it cures slowly (which means it is prone to sagging and incorporating bubbles). These problems can be solved by thinning the varnish with mineral spirits. The thinner material can be applied more easily and the higher ratio of solvent speeds the curing. Thinned varnish is known as "wiping varnish". The usual ratio of thinner to varnish is 50:50. Wiping varnish became popular with amateur woodworkers (including woodturners) in the late 1960's and early 1970's because of its ease of use and durable finish. Unfortunately, marketers have frequently mislabeled this finish, examples of products that have been mislabeled include; Formby's Tung Oil Finish - wiping varnish, has no tung oil Old Masters Tung Oil Finish - wiping varnish, has no tung oil General Finishes Arm-R-Seal - wiping varnish General Finishes Oil and Urethane Top Coat - wiping varnish, has no oil General Finishes Seal-A Cell - wiping varnish General Finishes Salad Bowl Finish - wiping varnish Others did combine oil (usually linseed oil) with varnish to create an oil/varnish blend (typically with equal parts of varnish, oil, and mineral spirits). Examples of these include; Watco Danish Oil Deft Danish Oil Finish Minwax Antique Oil Finish Olympic Antique Oil Finish Varathane Natural Oil Finish It is easy to test the finish you wish to use to determine whether it is a wiping varnish or an oil/varnish blend. Pour a small amount of the finish on a non-porous surface such as

5 glass or metal and allow this "puddle" of finish cure. If it cures smooth and hard, it is varnish only. If it cures somewhat soft with a wrinkled surface, it is an oil/varnish blend. Anyone can make their own oil/varnish blend by mixing separate oil, varnish and solvent together in the typical 1/3 to 1/3 to 1/3 ratio. If you want a thicker finish you can reduce the amount of solvent. If you want a harder finish you can reduce the amount of oil. Wiping varnish and oil/varnish blends are applied like oil finishes; wipe them on the finish surface with a cloth, let stand for 10 minutes or so, and, then wipe off any residual finish that has not soaked into the surface or evaporated. Then let dry overnight. This approach can be repeated several times to build up a thick finish coat. Advantages of Wiping Varnish: Thins easily with mineral spirits or turpentine Applies easily by wiping-on Run-free, brush-mark free and air bubble free application Very durable and protective against moisture penetration and resistant to scratches, heat and solvents Disadvantages of Wiping Varnish: Not as easy to repair or strip as Shellac or Lacquer Not as dust-free immediately upon application as Shellac or Lacquer Lacquer finish was originally produced centuries ago using a resin from the Asian Sumac tree found in Southeast Asia. Today, the finish is produced entirely by using cellulose nitrate combined with either an alkyd or maleic resin and an oil-like plastizer. The ingredients are dissolved in lacquer thinner composed of numerous solvents with different evaporation rates and which chemically combines the ingredients. Lacquer is only available commercially. Lacquer is very akin to varnish with one exception, the solvent used for thinning and clean-up. It cannot be thinned or cleaned-up with mineral spirits. It can only be thinned with lacquer thinner which is toxic and can cause health problems from exposure. The most unique characteristic of lacquer is resistance to runs and sags when applied to vertical surfaces. Used full-strength this finish dries hard, nearly as hard as varnish. Lacquer can also be thinned 50:50 with lacquer thinner to produce an almost invisible finish.

6 A closely related finish that has excellent applications to woodturning is water-based finish. Unlike true lacquer, this water-based finish is an emulsion of acrylic polymers, and sometimes acrylic/polyurethane resin.. The finish cures by the evaporation of the water in the finish solution to form a hard film. Water-based finish is the only colorless finish available for use on wood. This finish is available in spray cans for application in thin coats. Wax has been around probably as long as oil as a finish for wood. However, like oil, wax is not a durable finish and works best as a polish producing a sheen on the surface of the wood. Unlike oil which penetrates the wood, wax is a surface applied material. Wax is derived from animal, vegetable and mineral sources, and some are produced synthetically. Difference among the waxes comes primarily from differences in hardness. The higher the melting point the greater the hardness. Hard waxes (such as carnauba) melt at approximately 180 F and soft waxes (such as parafin) melt at approximately 130 F. The best melting point for a wax polish is about 150 F as found in beeswax. Any harder than that it is very difficult to buff-out and lower than that the wax will smudge rather than shine. Commercially available liquid and paste waxes have a melting point of about 150 F once all the solvents evaporate. Wax is not a good finish since it cannot be built-up sufficiently to protect against water. Water, even a drop, left on a waxed surface for even a very short time penetrates the surface to the wood below and raises the grain causing the spot to turn white. However, wax can be a finish choice for decorative objects which are not handled. Your wax of choice is applied by hand making sure that only a very thin coat of wax is applied. The last step is to vigorously buff the finished surface to produce a bright shine. As the shine dulls it may be re-buffed. When re-buffing does not produce a shine it is time to apply another coat of wax. Don't worry about wax build-up since the solvents in the new coat of wax dissolve any remaining wax and combine the old with the new. Apart from the waxes used as polish there is a finish system that can be used. This finish system utilizes waxes of varying degrees of hardness, some mixed with abrasives, and the final application being pure wax. The compounds and wax are applied to the piece using a series of spinning cloth wheels, each used with a specific compound or wax. The abrasive compounds are applied first to smooth the surface and the wax-only last. Heat generated by the buffing produces a hard bright shine.

7 Oil added to Finishes: Oil can be added to Shellac and Varnish to produce a "softer", more subtle finish. However, adding oil to these finishes also reduces the inherent durability of the finishes. Depending upon the appearance of the finishes desired, oil can produce an attractive finish. Sanding: A thorough sanding is the last step before applying any finish, or, should we say, the first step in the finishing process. Sanding involves progressive sanding of the entire surface of a completely turned piece beginning with #100 grit or #120 grit and proceeding through grits #150, #180, #220, and #320. Depending on the quality of the finish desired one may continue on through grits #400, #600, #800, #1000 and so on. Three things to keep in mind; do not use worn-out abrasive (it will only burnish the surface to be finished), thoroughly clean ALL dust produced by each abrasive by at least brushing the surface, but preferably, vacuuming, and, reverse the direction of turning between the use of each abrasive to pick up raised wood fibers. Sanding Sealers: A thinned coat of any finish will work as a sanding sealer since it seals off the wood pores. Make sure that the thinned coat of sealer is completely dry before beginning sanding. However, sanding with oil requires care in making sure the surface is clean of sanding material since oil doesn't really dry. Shellac sands better than varnish or lacquer which can "gum-up" on the surface. Commercial sanding sealers have detergents added to prevent gumming and to produce dust. The detergents in commercial sanding sealers can affect the quality of the finish applied following their use. Finishing Systems of the Pros: Many professional woodturners have developed "proprietary" finishing systems that they swear to achieve "the best" finish. Keep in mind that any finish system is based upon the finishes presented previously. If you look carefully at the description of any of the proprietary systems you will discover any, or many, of the following: A specific sanding process has been followed, or, A combination of sealing and sanding process has been followed,

8 A ratio of the finish and its solvent has been developed to suit the personal and environmental conditions application of the finish, The number and sequence of finish applications has been developed to suit the tastes of the woodturner, e.g., combination, if any, of finishes, number of applications, sanding or not between applications, polishing, or not, as a last step in the system. With the information provided in this brief summary you should be able to experiment and develop a finishing system that satisfies you. A Quick Comparison of Finishes: Groups Reactive Evaporative Coalesing Finishes Varnish Shellac and Lacquer Waterbased Moisture Resistance Scratch Resistance Very Solvent, Heat, Acid, Alkali Resist. Poor Poor Ease of Rubbing to Even Sheen Poor Ease of Dust-Free Applic. Poor (It should be noted that while oil provides very good moisture resistance, it performs poorly in all other categories.)