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Furniture finishing techniques

An overview

I am not a finishing expert... Not a Jeff Jewitt, expert on applying finishes, nor an expert on finish chemistry like Chris Minick or Jeff Weiss. But I build custom furniture, primarily indoor furniture. And I have developed some feelings on finishes - strong feelings. These notes are based on my experiences, many Internet forum discussions, discussions in our woodworking club, and in demos at the local Woodcraft store. And these finishing techniques have changed, since I started woodworking full time, and the finishes themselves have changed. I also don't have the luxury of a separate finishing room or a totally clean shop where pieces with slow drying finish can rest and cure without dust.

In addition to this overview, you can jump ahead to sheen and rubbing out the finish, lots more on lacquer, and spraying techniques.

Outdoor furniture

Everyone wants a finish that looks good, wears well, and protects the underlying material for many years, without maintenance. Sorry. No miracles. Or if you have a miracle, let me know. The finest marine varnishes have to be renewed once or twice a year. The most permanent deck paint or stain only lasts a few years. So I am down to two suggestions for clear outdoor furniture finishes:

Indoor furniture

There are countless options, many of which are combinations of other finish, or additives to a common finish - many seem to exist primarily to create a unique product (revenue potential for a more or less famous woodworker). This web page will only generally discuss oil finish and film finish. Oil finish where the finish soaks into the wood (even though there are many types of oil). And film finishes, where the finish forms a protective layer on, rather than in, the wood.

Oil Finish

This finish is wonderful... Sand to 400 grit or finer, apply a few coats of Boiled Linseed Oil (BLO), Tung Oil, or other oil, after 10 minutes or so for it to "soak in" remove the excess - wipe each coat "dry" - then let it cure for a day or more. Use steel wool, if necessary, to be sure everything is perfect between coats. Repeat as long as the oil soaks into the wood. Some people thin the oil so it soaks in better, but then may need to do more coats. After 3-10 coats (and many days of drying), wax with paste wax furniture polish. It brings out the best in the wood, and feels absolutely wonderful.

Of course, the oil has soaked into the wood, and the wax provides little surface protection, so dirt and body oils accumulate in the finish. One fine furniture store explained that, after a few years, you should remove the wax, dirt, and any oil on the surface with steel wool, apply new oil, and wax. Sure, every homeowner should refinish your furniture every three or five or ten years. Not me. Some oil finishes primarily soak into the wood, but also leave a thin surface film - a cross between oil and film finish. The surface oil provides a little extra protection - perhaps taking the place of the wax.

Film finishes

Unlike oil, a film finish such as shellac, varnish, or lacquer, provides a hard layer on the surface of the wood, so protects the wood, and allows dirt and body oils to be cleaned off the surface. With basically all film finishes, the film will fill and hide the fine scratches in the wood left by sanding, so you only need to sand to 150 or 220 grit; not to 320, 400, or finer, as you would with an oil finish.

Film finishes are either evaporative or reactive. Reactive finishes, like paint, varnish, and polyurethane, react with the air and form a film - we call it drying, but it is a chemical reaction. If you put on two coats of paint or other reactive finish, you have two layers of the film finish.

Evaporative finishes, like lacquer and shellac, are dissolved in a solvent. When they dry the solvent evaporates and leaves the finish, but if you add more solvent, it dissolves again. Finger nail polish is a good example... it is lacquer. Put it on, the solvent evaporates, and you have a hard finish. Put on another coat, which includes solvent, and it blends with the previous coat, and you have one layer of finish on your nails, you don't peel it off a layer at a time like an onion. To remove the finger nail polish, use a similar solvent (either finger nail polish remover from the drugstore with perfume at a high price per ounce, or lacquer thinner from the hardware store with nasty odor and a low price, sold by the gallon.)

Why do you care whether a finish is reactive or evaporative? With fine furniture, most people want to rub out the imperfections and smooth/polish the surface with progressively finer abrasives. With the multiple layers of a reactive finish, you might break through a layer of the reactive film finish. That leaves a "witness line" around the hole in the outer layer (and the deeper layers if you go through more). I have never succeeded in hiding a witness line. If I sand through a layer and get a witness line, I just sand all the finish off and start over. But if you have an evaporative finish where a new coat of finish melts into, or dissolves into, or burns into, the previous coat (burn-in is the term most often used in furniture finishing) you no longer have separate layers. You can rub away until you have removed all the finish, and if accidentally reach wood, you can add more (spray on a patch) it will burn into the existing finish. This is a huge feature to custom furniture makers. I want a evaporative finish when I am planning to rub out the finish. (Commercial shops don't know what you are talking about because they cannot afford to rub out a finish.) Or if you must rub out a reactive film finish, to get rid of the specks of pollen or whatever, you need to be very careful - sand very gently so you only work with the top layer. If you have to sand between coats "to seek forgiveness for that drool" or whatever, you can be aggressive with an evaporative finish, but need to be super gentle with a reactive finish so you only do the top layer, and don't create a witness line.

Shellac

This is a historically traditional evaporative film finish. It is excreted by the Lac bug, collected, and dried. The dry crystals are dissolved in alcohol. It dries by the evaporation of the solvent (alcohol), without a chemical reaction to form the film, which means that the alcohol can re-dissolve the finish. French polish is the art of applying shellac in a way that creates a fine finish without separately rubbing out the finish. Shellac is arguably soft, and definitely subject to water and alcohol damage (the white glass rings on tables are often in shellac).

Shellac chemistry is simple... start with the dry crystals in your choice of natural colors (blonde - light yellow, amber - medium yellow, garnet - almost brown). Dissolve in alcohol ("1 pound cut" is 1 pound of crystals in a gallon of alcohol (very thin), "3 pound cut" is 3 pounds of crystals in a gallon of alcohol (pretty thick). Add alcohol to make it thinner. Let it evaporate to make it thicker. The alcohol is most often denatured alcohol (a poisonous variant so you can't drink it, which means so you don't have to pay liquor tax on it, but Vodka works, and moonshine works too). Experts point out that alcohol absorbs water from the air, so, once mixed, shellac starts to age by absorbing water - only mix what you expect to use in the next weeks or months. You can buy it pre-mixed (Zinsser is the primary brand, and their quarts of shellac are 3 pound cut). There is a date of manufacture on the can, and experts suggest discarding it after one year.

Natural shellac contains wax in addition to color. But you can buy dewaxed "white" Seal Coat from Zinsser, that is a "2 pound cut" of shellac with the color and wax removed (no explanation why they removed the name "shellac"). Shellac, especially dewaxed, is a great finishing tool - the wonder drug of finishing. It sticks to practically anything. It brings out the color and grain when applied directly to wood. And practically any finish sticks to it. If you have a mystery finish you need to cover, put on a coat of dewaxed shellac (such as Zinsser Seal Coat), and then proceed with regular finishing. As a tool - a primer or intermediate layer in finishing, I love it, but not as a final finish.

Some of my friends have had problems with Zinsser - they recommend mixing the shellac crystals rather than buying it pre-mixed. I heard that Zinsser made their shellac slightly acid for longer shelf life - that acid may be causing the problems my friends experienced if they used a heavier coat of shellac. I only use thin coats of shellac and have not had a problem.

I find shellac very hard to clean off tools... I have never been able to clean a brush well enough to use for something else. I put a small amount of shellac in a tall glass jar, and leave the brush in the jar with the shellac, ready to use as my finishing first aid kit. Since it is solvent based, the brush never dries or forms a "skin" - if it gets thick, just add more alcohol. Some people suggest letting a shellac brush stiffen, then just put it in the shellac to soften 30 minutes before you want to use it. Some forum users suggest that household ammonia allows them to clean shellac out of guns and brushes.

Varnish

This is a traditional high quality film finish, readily applied by brush. It is a reactive finish, so "drying" is a chemical reaction that creates the film, rather than simply the evaporation of the solvent. (If you have added paint thinner - it evaporates, but the varnish doesn't cure and "dry" until the reaction that makes the film. After the varnish is dry, adding paint thinner has no effect. Paint thinner is not a solvent for varnish.) The layers are hard and durable, but each layer remains separate (a challenge if you "rub out" the finish). With a slow drying time (as needed for most brush-on finishes), it is subject to contamination for longer than I can keep my shop clean. Varnish really needs a clean room, a high quality brush, and a skilled operator to apply well. I haven't got the clean room, the time, or the skill.

Some people thin varnish with up to 50% paint thinner, and wipe the thin finish on with a rag or sponge brush. It dries fast, rarely has flaws like dust or drools (so doesn't need to be rubbed out), but needs a large number of coats to build a comparable protective film.

What is a "conversion varnish"? The varnish has extra chemicals that causes the finish film to be harder than usual, and in some cases cure faster than usual. The spray is more toxic, so should be done with a respirator (all spraying should be, but this is more equal than others). Some argue that it should only be sprayed by professionals.

Polyurethane

"Poly" is chemically different than varnish, but has many of the same characteristics. It is easily brushed on, dries faster than varnish, but still slowly enough to attract every dust particle within miles. If subsequent coats are applied within hours of the previous coat, the layers bond together (but not full burn-in to a single coat); if the previous coat is totally cured, the surface must be sanded (scratched) so the subsequent coats will adhere properly. And if you are going to rub it out, it is easier to do so before the finish has totally hardened.

Poly is very hard - so hard that it is very difficult to scratch. Therefore I love to use use poly on floors. But years ago I put poly on our "kitchen" table. Hard is also brittle. I don't know how they did it, but my son and his friends, as they were growing up, managed to chip the finish on the table - create divots in the surface. In my personal opinion, polyurethane is too brittle for furniture, although lots of people use it for furniture.

Lacquer

This is a traditional fast drying spray finish. It is a solvent finish that can be applied in multiple layers, that will "burn in" and become a single durable layer. If you need to repair or "rub out" the finish, there is no worry about rubbing through a layer and leaving a "witness line" where you transition from one layer to another. It dries fast enough that there is little or no problem with dust, and numerous coats can be applied in a single day. It also dries so fast that it is difficult to brush - most people consider it a spray-only finish.

Lacquer wears well on indoor furniture, and has passed my tests (spill wine on it, then set the wine glass in the spill for a few days, so it doesn't evaporate, and check for damage... repeat for other materials). For many years it was used for cars, and is still used in finger nail polish. Catalyzed lacquer includes extra chemicals that make the lacquer even more durable - less susceptible to chemicals. The Kitchen Cabinet Manufacturer's Association (KCMA) does not recommend plain solvent based (nitrocellulose) lacquer, but does approve catalyzed lacquer - with catalyst added at the factory or store (PreCat) or at the time of use (Post Cat). Once the catalyst is added the shelf life is limited - typically weeks to a year. Acrylic lacquers are a high tech finish that are even more durable. This acrylic Lacquer is the finish that I normally use for indoor furniture.

Combinations of finish

A sanding sealer is a fairly soft first coat to seal the pores of the wood. It is then sanded, even down to the bare wood, to create a super-smooth finish on which to build the further coats. If I start with shellac as a sealer, I don't normally use other sanding sealers. Rather than shellac, my current favorite sealer is EM1000 sealer from Target Coatings - a little more expensive, but it goes a long way, brings out the natural color in the wood, and as a water based finish is easy to use and clean up.

The primary finish I use to build the multiple coats for a durable finish is Target EM6000 water based production lacquer. Common knowledge is that water base finishes cannot burn in, since they don't work by evaporation of the solvent. In general that is true, but don't tell Target Coatings. They have managed to get burn in with EM6000, previously called USL or Ultima Spray Lacquer. I don't know how they do it, and probably wouldn't understand if they explained it, but it works. The water evaporates, which allows a chemical reaction to occur, which allows the lacquer to burn into previous coats of lacquer, just like the regular solvent based lacquers. Miracles happen here.

If I need an especially hard or durable finish, such as a tabletop, I put on a final coat or two of Target EM8000cv. As a conversion varnish, it is a reactive finish, so I have to be very careful rubbing it out. I have also used Target EM9300, which is also recommended by some as a super-hard top coat.

Color

I strongly prefer showing the beauty of the wood without artificial color. But there are times when it is appropriate, normally applied under the final finish.

Dye

Dye changes the color of the wood fibers, rather than applying a color to the surface. The character of the wood is now different - the grain may be emphasized or masked, but it is not covered by the dye itself. Dyes may be water based or solvent based - there are arguments for both - but they are always covered with a film finish that makes them permanent. Bleach is often used to remove a dye.

Stain

Stain is a transparent paint that is applied to the surface of the wood. Sanding scratches and rough surface cause more of the color to "stick" in an area; fine sanding of the underlying wood reduces the amount of stain that "sticks." End grain (or areas of uneven grain in the wood) become darker, so extra fine sanding of the end grain can reduce the darkening. If I expect wood to become "blotchy" one solution is to sand to a very fine grit (400 or finer), and then lightly and evenly "scratch" the surface with a coarser sandpaper (such as 220 grit) before applying the stain. The more traditional solution is to apply a coat of very thin shellac or other sealer, then sand before applying the stain.

Many commercial "stains," like the popular MinWax by Sherwin Williams, often sold at discount stores, are actually a mixture of stain and dye. The final character of the stain depends as much on the wood and wood preparation, and the application techniques, as the stain itself - matching another piece is a special art. Paint remover is often used to remove stain.

Toner (and glaze)

The clear "final" coats of finish can be tinted, making it a toner, to add shading or color tone. Note the color is in the finish rather than in or on the wood - a great technique to fine tune a finish. However, some really cheap furniture is finished with all the color in the toner, none on the wood, making the furniture almost impossible to repair.

Toner is different than a glaze, which is color (often wiped on by hand) to darken the corners, bring out the shadows, and perhaps to "age" the piece. The glaze is then covered with clear finish to protect it and the other layers of finish.

Some of the traditional finishes such as shellac and varnish were amber/yellow, so the newer "clear white" finishes are sometimes tinted, like a toner, to have the more traditional "hint of yellow" color.

Paint

Paint is intended to cover and hide the wood. This allows use of materials that aren't naturally attractive - for example, MDF makes an excellent floating panel in a painted door. Paint is far higher tech than many realize. The beautiful black grand piano is not ebony but is typically maple wood coated with black paint under clear finish coats. Ceiling paint has better hiding power and reflection than wall paint, but little or no resistance to abrasion (wear). Wall paint is designed to withstand wear (touching and cleaning). Floor paint (ok, deck paint) is very hard - scratch resistant. House paint is very flexible, so it can expand and contract with the weather, thus is soft and less abrasion resistant. Primer has good hiding power and has a "tooth" to hold the following paint, but that tooth also holds dirt and doesn't wear well - it needs something over it. Don't assume that leftover house paint will be fine as a primer coat on your painted bookcases - it is far too soft.

Many types of antique furniture used different types of wood for different parts - the curved back of a Windsor chair is usually steam bent oak, the deep seat profile might be carved from pine, and the turned spindles maple to show the sharp detail. Paint, such as "milk paint" was used to hide the differences in the wood. "Milk paint" is nasty stuff, but is still available if you are making antique reproductions. I have recently met several people who believed that milk paint was the preferred paint for all furniture. Think enamel for modern furniture, and leave the milk paint for antiques and reproductions.

Jump ahead to the page on lacquer, the page on rubbing out the finish and achieving the desired sheen, or to the page on spraying, or jump back to the index of woodworking tips.