Choosing A FinishPosted by Jewitthome on May 8, 2000 in Articles | Comments Off on Choosing A Finish
Copyright 2000 by Jeff Jewitt Not reproducible in any form, written or electronic, without permission
Undoubtedly, one of the continuing bugaboos for woodworkers is finishing. Though undaunted by complex joinery or intricate and precise machining, many woodworkers still cringe at the thought of applying a finish to their work. “What’s the best finish for my project” is one of the first hurdles to overcome.
Of the hundreds of products sold as finishes to woodworkers, all finishes can be grouped down into more manageable categories having general working qualities and degrees of protection. These groups are waxes, oils, varnishes, shellac, lacquer and water-based finishes. Different finishes have varying degrees of protection, durability, ease of application, reparability and aesthetics. Unfortunately there is no finish that excels in all of these categories. So choosing a finish is about trade-offs – a finish that excels in one category may fail in another.
As a professional refinisher, I routinely ask my customers a series of questions so I can determine the best finish for their furniture. They’re designed to give me an accurate picture of what finish is appropriate, and I accompany these with finished samples. I’ve modified these questions a bit and added a few so that woodworkers can determine what finish to use. When you choose a finish you should consider the following.
1. How will the item be used? Is it subjected to a lot of moisture, solvents, food, scrapes and dents? Or will it be primarily a display piece.
2. What is your skill level and work area – does it stay clean and is it heated and dry?
3. What do you want the wood to look like – do you want an “in the wood” natural look or a thicker finish that accentuates depth?
4. Will you be filling the pores?
5. What color do you want the finish to impart to the wood – do you want the finish to alter the color of the wood. Is yellowing an issue? Do you want to minimize the wood changing color as it ages?
6. Will you rub the finish out to the sheen you like?
7. Safety and health – are you sensitive to certain solvents, is flammability a problem, or are you concerned about the environmental impact of certain finishes
8. Toxicity of the finish – will it be used in food preparation?
Keep in mind that finishes are generally divided into two distinctly different “types”. Evaporative finishes are those finishes that dry or cure to a hard finish by evaporation of the solvents in the finish. These finishes are lacquer, shellac, and water-base finishes. Reactive finishes are finishes that cure by reacting with another component – either outside the can (like air) or placed in the can right before application. Examples of reactive finishes are oils, oil based finishes, and catalyzed lacquers and varnishes. General working qualities are similar within these groups like reparability and dry time (the exception being pure oils like tung and linseed oil)
This category comprises the durability of the finish and its resistance to water, chemicals and solvents (like alkaline cleaners and acidic foods), heat and scratches. As a rule, evaporative finishes are less durable overall than reactive finishes (with the exception of true oils). Waxes, shellac, lacquer and some water-base finishes can be damaged if exposed to water long enough. Shellac is not resistant to alkalis like ammonia. Most of these products will scratch easily, however, they rub out well (that’s the flip side of scratch resistance). Waxes are surprisingly resistant to acids and alkali’s, however, aside from this they are the least durable. Of all the evaporative finishes, lacquers like nitrocellulose and the acrylics (both water-based and solvent ) fare the best for overall durability.
Reactive finishes (other than pure oils) are much more durable than the above finishes. Oil-based polyurethane is the most durable finish that you can apply by hand, while catalyzed lacquers and varnishes are the best for spray application.
Obviously, your degree of skill, the environment you have to work and the tools you have access to play a part in choosing the best finish. If the environment in which you have to work is cold, or there’s constant sanding dust in the air from running machines, the evaporative finishes are the easiest to work with. Because these finishes dry fast, dust falling in the finish does not pose as great a problem as slow drying finishes like varnish. Shellac and lacquer are the least temperamental when it comes to cold temperatures, and can be modified with additives (retarders) for hot and humid conditions. Oils and oil based products dry slowly in cold temperatures and humid conditions, and dust is always a problem as they remain tacky for a long time and dust becomes embedded in the dried film.
Spray equipment requires a larger budget, and in some cases requires equipment to exhaust the overspray. There’s also a learning curve in spraying, so it may take a bit of practice before decent results can be expected. However, the speed and subtle control over applying finishing products is well worth it for most, even with modestly priced equipment.
This category has to do with how you want the wood to look. Do you want a natural, “close-to the-wood” finish. Or do you want an elegant, deep, glass-smooth finish that accentuates depth and luster. Is the color of the finish a problem, or will yellowing of the finish be a problem later?
Traditionally, woodworkers have turned to the oils (tung and linseed), wax, or oil-varnish blends (like Watco) for a natural finish. These easy-to-apply finishes do not form a hard film when dry, so they can only be built up to a certain point on the surface of the wood. However, any finish can be used for this effect. As long as you don’t build up the finish beyond several coats, shellac, lacquer, varnishes and catalyzed finishes can all be used for natural-looking finishes.
Hard, film-forming finishes like shellac, lacquer or varnish have to be used if you’re going for a filled-pore, deep, lustrous finish. These finishes also are used when complex coloring options like toning and glazing are used.
The color and the penetration of the finish itself may be an issue. Orange shellac or tung oil/phenolic resin varnish both may have a color that’s too dark for woods you may want to keep as light as possible. In addition many finishes deepen or darken the wood surface. In most cases this is desirable, because it kicks out depth and increases luster. However, you may want to downplay the deepening effect. Some delicate figured woods may turn to a muddy appearance with an oil finish. As a rule, oils and oil based varnishes deepen color of the wood and increase luster the most, followed by solvent based lacquers and shellac. These finishes “wet” the cells of the wood – penetrating into the surface. Other finishes tend to lay on the surface and do not penetrate which has the effect of keeping the wood lighter. These finishes are most water based finishes based upon polyurethane or acrylic, waxes, and some catalyzed finishes.
The “plastic” look that’s sometimes ascribed to finishes like polyurethane and catalyzed lacquers has to do more with incorrect application of these finishes than the finish themselves. On open-pored woods, applications of thick, high solids varnishes and lacquers results in a “soupy” look to the pores, a result of the finish bridging the pore rather than flowing into it. Proper application of these finishes by thinning them results in a more attractive look. My favorite method for oil-based polyurethane application is to thin the finish 50% and wipe it on.
Yellowing of the finish is a problem over white and painted finishes (yellowed lacquer over a blue paint will turn it green) or if your intentions are to retain the natural color of the wood as much as possible – as in maple or birch. Acrylic based finishes, both water and solvent based are the best choice for non-yellowing finishes. Waxes also will not yellow as well as some catalyzed lacquers and varnishes.
Safety / Environmental
Solvent based finishes like varnish and lacquer contain a good deal of organic solvents, which create an environmental and health impact and they are flammable. If this is a problem for you, use a water-based finish to eliminate the fire problem and mitigate the environmental and health impact. Oils are a surprisingly good alternative to solvent based lacquers and varnish, they are 100% solids (no solvents), and come from renewable resources however, oil-soaked rags must be disposed of carefully. Shellac is also a good alternative. The solvent for shellac, ethyl alcohol, is distilled from corn and for most people, the fleeting odor isn’t objectionable.
All finishes are non-toxic when fully cured, despite what you may have read or heard. Once the solvents are evaporated, the cured film is safe enough for contact with food. This does not mean that the finish itself is safe to gobble up. It means that additives such as driers or plasticizers are encapsulated enough so that they do not migrate to what you’re eating. For edible finishes, wax and shellac are the only ones I’m aware of (which is why apples and candy are coated with these)
Spraying wastes a good deal of finish and the organic solvents are dispersed into the air. Brushing or wiping on a finish is a practical, though less speedy alternative
The two drying oils used in finishing are linseed oil and tung oil. Other drying oils have been used, but these two are easily available and inexpensive. In an effort to distinguish these two products from other finishing products hyped as oil finishes, we call these true oils. The conversion of the liquid oil to a solid is through a process called polymerization.
Linseed oil is available in several forms. Linseed oil in unrefined form is called raw linseed oil and is rarely used on wood because it dries very slowly. Our ancestors realized this but found that if the oil was heated (boiled), the resulting product was thicker and dried quicker. Though real heated oil is available (called heat-treated or polymerized oil) boiled linseed oil made today uses a slightly different process – the raw oil has chemical additives which speed up the drying time and oxygen is bubbled through the oil. For wood finishing purposes boiled linseed oil should always be used.
Tung oil is derived from the nuts of certain trees indigenous to Asia and other parts of the world. Tung oil is available in pure, unrefined form or heat-treated in a form called polymerized tung oil. The heat-treating process makes the oil a tad bit more durable and speeds up the dry rate. It also minimizes the tendency of tung oil to “frost” or dry to a whitish, matte appearance. It is a bit paler in color than linseed oil and has better moisture resistance than linseed oil.
Both linseed and tung oils are penetrating finishes – which means that they penetrate into the fibers of the wood and harden within the wood. These are the easiest finishes to apply – they are wiped on, allowed to penetrate and the excess wiped off with a rag. They are generally not built up to a surface film like varnish or lacquer because the film dries too soft.
Varnish refers to finishes that are made from drying oils like linseed or tung and tough, durable synthetic resins. Some varnishes are simply solutions of the resin and oil, like phenolic resin varnish. Others like alkyd varnishes and polyurethane are really drying oils like linseed, tung or soya oil that’s modified with chemicals called polyols and acids. This forms an oil/resin structure called an alkyd. Because alkyds contain residual oil they dry by the same process as true oils – polymerization, but the resin makes the finishes more durable than oils. In fact, oil-based varnishes are the most durable finish that can be easily applied by the average finisher, surpassing most of the other finishes in water-resistance and resistance to heat, solvents and chemicals. There are three general groupings of varnish. Those based upon a high percentage of oil in the finish are called long oil varnishes. Medium length varnishes have a lower percentage of oil and short oil varnishes even lower. The ratio of oil to resin affects the hardness of the varnish. Long oil varnishes are also known as marine varnishes, spar varnishes or just plain exterior varnishes and are elastic and softer than short oil varnishes. Short oil varnishes are for industrial use and require baking to cure properly. Medium and long oil length form the basis of most wood finishing varnishes.
- Alkyd varnishes form the basis of most interior varnishes and have good all around protective qualities. Linseed and soya are typically used, and soya produces a lighter colored varnish and less prone to yellowing over time. It dries slower than linseed oil alkyds.
- Phenolic/tung resin varnishes are very good exterior varnishes and most spar varnishes are made from this, or in combination with an alkyd.
- Polyurethane is really a name for uralkyd, an alkyd that’s made by substituting a portion of one of the chemicals normally used with an isocyanate, producing urethane resin within the structure of the alkyd. The incorporation of urethane makes the varnish tougher, more scratch, heat and solvent resistant (think of urethane like the soles of your shoes). However, because the uralkyd molecule is large, some urethanes may exhibit a haziness if applied too thickly and are optically different from lower molecular weight resins like shellac and lacquer.
Varnishes are typically applied with a brush, though highly thinned varnishes and gelled versions can be applied with a rag and are called wiping varnishes.
Oil/varnish blends are products that are made from, you guessed it, oil with some varnish in it. The resulting mixture has some of the application ease of true oils but pick up some of the protective qualities of varnish. Watco™ Danish oil, teak oil, Nordic oils, and a host of the other finishes sold as oil finishes fall into this category. It’s impossible to ascribe general protective qualities to these products as the amount of oil to varnish is not disclosed by manufacturers, but they will dry a bit harder than true oils, build quicker and fewer applications are needed. The thing to remember is that as the amount of oil increases, flexibility increases but at the expense of hardness.
While most people think of shellac as a liquid you buy in a paint store – in reality it’s a natural resin that’s derived from the secretions of the Lac bug – an insect that feeds off trees indigenous to India and Thailand. The secretion is in the form of a cocoon – these are gathered from the trees and eventually refined into dry flakes which are then dissolved in alcohol to make the shellac solution which winds up in cans at the store.
Shellac is available in several forms. You can buy it pre-mixed or you can buy it in flake form and mix it yourself with denatured alcohol. The pre-mixed variety is available in orange (amber) and clear (which is shellac that’s been bleached). Dry shellac is available in dry form which you mix with denatured alcohol in the amount you want. It’s available in a wider variety of color and wax content than the pre-mixed variety which always contain wax. Wax decreases the resistance of shellac to water and prevents some finishes from bonding to it.
The term lacquer generally means a fast drying glossy, hard finish based upon flammable solvents. It is still considered by many professionals to be the best all-around finish for wood as it is fast-drying, imparts depth and richness, has moderate to excellent durability (depending on the type used) and rubs out well. There are several different types of lacquers with different performance characteristics and they are
- Nitrocellulose lacquers
- Acrylic modified lacquer (CAB-Acrylic)
- Catalyzed lacquers
Nitro Nitrocellulose lacquers are the most common lacquer you will find and if the label on the can says lacquer—it’s this type. These finishes are composed of an alkyd/nitrocellulose resin dissolved and then mixed with fast-evaporating solvents. They have moderate water resistance, but are sensitive to heat and certain solvents. The biggest drawback is their tendency to yellow over time which looks bad on light colored woods and white finishes.
Acryl Acrylic modified lacquers are usually based upon a mix of a non-yellowing cellulose resin called cellulose acetate butyrate (CAB for short) and acrylic. They have the general properties of nitrocellulose lacquers with the exception of being absolutely water-white – meaning they will not impart an amber shift when applied to light woods and will not yellow over time.
CatalCatalyzed lacquers were developed to bridge the gap between the application characteristics of nitrocellulose lacquers and the performance and durability of varnish. These complex finishes are composed of urea/formaldehyde or urea/melamine and an alkyd that has some nitrocellulose resin added to make it handle like normal lacquer. An acid catalyst is then added to initiate a reaction which forms a very tough, durable finish. They are available in two versions. One is a two-part system that must be mixed in the proper ration and then applied to the wood. These lacquers have a short pot life (the time in which it can be used) and must be mixed in the proper ratio. The second type called – pre-catalyzed lacquer – has the components premixed – either by the manufacturer or at the store when you buy it. Catalyzed varnishes are usually water-white and non-yellowing because nitrocellulose resin is not used.
Water-based finishes are based upon the same components as the finishes we discussed above – notably urethane (usually made without the oil), alkyd and acrylic. But the flammable and polluting solvents have been partly replaced by water. The chemistry involved in doing this is a little complex because usually these resins do not have a natural affinity for water unless they are chemically modified or “forced” to combine with water in a chemical mixture called an emulsion. Emulsions require the addition of chemicals called surfactants and special solvents generically called glycol ethers.
Water based finishes are usually based upon an acrylic resin which is sold as water-based lacquer or a acrylic/urethane mixture which is sold as water-based polyurethane or “polycrylic” or some other acronym. As in varnishes – the addition of the urethane makes the resin tougher and more scratch resistant – but that’s where it stops. Water-based urethanes do not have the solvent and heat resistance of the oil-based counterparts. The recent decade has also seen the introduction of water-reducible alkyds and urethane-alkyds (uralkyds) which are virtually identical to solvent based alkyds except they thin with water. These waterbased finishes have the pleasing amber color of solvent based products and they look better on woods like walnut, mahogany and cherry than standard acrylic and urethane waterbased finishes.
Why Finishes Yellow
Most finishing resins yellow over time and nitrocellulose lacquer and oil-based polyurethane are two of the worst. The reason they do this is because of exposure to light and air. Visible light – particularly in the ultraviolet region breaks the electronic bonds that holds the finish molecule together – eventually forming new chemicals in the finish that are yellow colored. Other factors such as high heat and moisture may have an effect as well as contact with rubbers or plastics that contain sulfur. The rubber protective bumpers on the base of a vase may actually create yellow impressions in a clear lacquer finish.
To minimize yellowing – try to control harsh sunlight if possible and never leave plastic or rubber items (like a vinyl tablecloth) on finishes (especially lacquers – both nitrocellulose and CAB-acrylic) for extended periods of time.
Suggestions for Choosing a Finish
So, you ask, what’s the best finish to use? It depends on how durable you need it to be and the other requirements discussed above, like aesthetics and you working environment.
The best all–around finish that looks pretty good and will hold up to harsh abuse is oil based polyurethane if you apply finishes by hand, and catalyzed varnish and lacquer if you spray. Water base versions of these finishes are almost as durable, but lack the depth and warmth of the oil based versions. In a warm, damp environment like a kitchen or bathroom, long oil based poly or varnish are good choices. Floors are a good choice for oil poly, but the smell and dry time may be an issue, in which case water base poly works best. Most professional floor finishes now use water base poly for these reasons.
On furniture that won’t get harsh treatment from feisty two-year olds or be exposed to water, consider lacquer, shellac, oil, or oil-varnish blends. The benefits of lacquer and shellac is that they dry quickly. Freshly made, dewaxed shellac is a surprisingly durable finish that’s under-rated.
There is no “best” finish for items that are in contact with food. No finish at all is an option, and there are natural nut oils and mineral oil (available at drugstores) that re-new the appearance of wood without adding harmful solvents. My own preference for items like baskets and bowls is several coats of dewaxed shellac. It seals the wood against water and can be wiped clean with a damp cloth (not dripping). I also suggest shellac for toys that children may possibly put in their mouths.
The best clear finish for exterior use is oil based marine varnish. These tung oil/phenolic resin varnishes hold up the best, but very harsh conditions (like direct sunlight) may require sanding and recoating every two years. If you go to a marine store, versions of these with special additives to protect against sun are available, and you’ll squeeze maybe an extra year or two out of them. The downside is that they are double and sometimes triple the cost.
Finally – if you want a finish that doesn’t yellow, get a water-base finish based upon acrylic or non-yellowing polyurethane.