from bare wood to ultimate use
for the serious woodworker
Intro
There are many pages on this site concerning different parts of the finishing process - different types of finish, spray techniques, and so forth. This tutorial will try to pull the various parts together into an overall picture for a woodworker who is building furniture. For a deeper understanding be sure to see
Finishes - indoor and outdoor, and the different materials
- this overview is not trying to replace any of the more detailed web pages.
Sanding
The initial step is ... duh ... to complete the project. But that isn't just putting the pieces together, removing the clamps, sanding, and finishing. You may need to give a few days for the glue to dry before you start finishing. What? Even though you may use glue that can be taken out of the clamps in an hour or two, and claims to be "dry" to full strength in an day, you may want to let your piece age for several days before you do the final sanding and finishing. If you have edge joined two boards (or a board to a sheet of plywood) and the glue is "dry," if you used a traditional wood glue, a lot of water was added as part of the glue. The wood around the joint has expanded. If you sand it flush in the first day or so, the water will continue to escape from the vicinity of the glue joint for several days, and the wood in the vicinity of the joint will shrink. It will create a "sunken joint." The area over a biscuit or domino will shrink and leave a "ghost" of the joining piece.
After the piece is totally dry (those extra couple days, at least on the primary surfaces), the whole piece needs to be sanded. If you will be using a film finish such as varnish or lacquer, that film will cover the wood completely, and will fill fine sanding scratches. Sanding to 150 or 220 grit is probably sufficient. If you will be using an oil or polish finish, where the actual wood can be felt as well as seen, you may want to sand to 320, 400 or finer. Note that "sanding to 400 doesn't mean just touching all surfaces with 400 grit sandpaper - it means perfecting the surface with each coarser grit before moving gradually to the finest grit. (That may seem obvious, but I had to referee a job where a professional builder told his men to sand to 320, but they just went over the entire surface lightly with 320 grit, not sanding properly through all the lower grits. The professional finisher who followed wasn't allowed enough time in the schedule to recover from the inadequate sanding.)
Some sandpaper and sealers leave chemicals on the surface (often stearate) that may interfere with the finish. One finishing forum suggests wiping after every sanding step with a 50/50 mixture of alcohol and water. Even though alcohol dries quickly and completely, residual alcohol may interfere with some finishes. Therefore I have a spray bottle with the mixture, spray a light coat on a clean rag, and use that rag to wipe the surface - like the old fashioned tack rags (that should not be used with new technology finishes).
Filling
If you are planning on a high gloss "grand piano" finish, you need to use a wood grain filler on open grain woods like oak, mahogany, walnut, and cherry. If you want the grain texture to show, skip this step. The one I use is at the left, but I am sure others would be fine. This is typically a paste-like substance that fills the pores of the wood (and minor sins like imperfect glue joints). It is thicker than latex paint, and weighs a LOT more - it actually fills the grain with a fine mineral. Slop some on the surface and rub it across the grain with a plastic putty knife. Since it is water based, it dries very quickly (smear fast). Once it is dry, sand down to the surface of the wood. As a mineral, it doesn't absorb dye, but it can be stained - however, I find the brown filler works on most wood and doesn't need extra color. The "neutral" is almost white, which creates an interesting effect, but less natural (to my eye) than a darker filler. Some people argue that the grain filler should be used after sanding sealer and dye, but I prefer using it before, so I can sand like crazy without worrying about removing some of the color.
Sanding Sealer
There are two purposes for sanding sealer. 1) to fill the pores (including those left by the grain filler, if any). And 2) to bring out (pop) the color of the wood. For years I couldn't believe you could or should use a sealer before a stain or dye (in this role sometimes called a conditioner). A thin coat (diluted 50%) of sealer lets the part of the wood that would soak up too much stain (dark and blotchy) satisfy the thirst. It then can be lightly sanded (if you put too much on) so the surface of the wood can absorb stain evenly. I have finally become a believer.
Solvent based dewaxed shellac is a good sealer and a good barrier between mystery surfaces (if you are refinishing), since it adheres to almost anything, and almost any final finish adheres to it. It does a good job of bringing out the color of the wood. The Zinsser "Seal Coat" above is a 2 pound cut of dewaxed shellac, meaning 2 pounds of shellac crystals were mixed with a gallon of alcohol. The clear pre-mixed shellac sold by Zinsser is a 3 pound cut (thicker). The sanding sealer should be thin (to get into the pores) so the shellac can be diluted with denatured alcohol (DNA) to a 1 pound cut (equal parts DNA and seal coat). A couple of my friends have had compatibility problems using premixed shellac like Zinsser Seal Coat, but have not had a problem starting with shellac crystals. Recently I learned that Zinsser made the Seal Coat slightly acidic to enhance shelf life. Since many water based finishes are alkaline, a heavy sealer of pre-mixed Seal Coat could create a problem. I have not had a problem, but I use a minimal amount of Seal Coat if that is my sealer of choice.
The other sanding sealer that I "normally" use is Target Coatings EM1000 universal sanding sealer. It is a water based sealer that dries quickly and is extremely easy to clean out of the spray gun. It creates a soft film that is easy to sand with minimal clogging the sandpaper. It sprays as easily as the other Target water based coatings (you can thin with up to 5% water if you wish, for penetration of any open grain).
After applying the sanding sealer, sand to a perfectly smooth finish. If you break through to bare wood, don't worry - the sealer is primarily in the wood grain. If the finish isn't perfectly smooth, you can add more coats of whatever sealer you are using, and sand again until you have a perfect base to build the finishes. The sealers are softer than the final finishes, making it easier to sand, but don't try to "build" the layers of finish with the soft sanding sealer - only use enough to end up with a very thin layer on the wood.
Color - stain or dye or neither
Stain is a color like very thin paint - you can still see the grain through most stains, but the large molecules of the color lie on the surface or in the pores of the wood. Dye actually colors the wood - very small molecules get into the wood, rather than in the grain between the wood fibers. Why is this important? If you used a mineral grain filler, the mineral doesn't absorb the dye, but it can be stained. How can you tell? Try stirring the very popular MinWax Wood Finish interior stain (made by Sherwin Williams but available everywhere from HomeDepot to WalMart). Your paint stick will be colored but the thick part is at the bottom. The stick is colored by dye, a liquid; the thick part at the bottom of the can is the stain that needs to be stirred in. This finish is a mixture of stain and dye.
Stain is often applied by brush or spray, then wiped into the wood, with the excess wiped off before it dries. Dye can be done in a similar manner. If you get it wrong, use bleach to remove dye, paint remover to remove stain.
Toner is applying color by putting a small amount of dye into the sealer or first coats of the finish (particularly lacquer which can be applied in many thin coats until the desired shade is achieved). The disadvantage of toner over stain or dye is that all the color is in the finish, not in the wood - dings to the furniture are harder to hide. The advantage is that thin layers can be applied until the perfect tone is achieved.
My goal is usually to use wood that has the desired color without adding any stain or dye. This is more natural, allows the wood to "show off" better, and is more easily repaired. However, even when I am being "totally natural" I sometimes spray a little toner to hide or de-emphasize sapwood or variations in the color of the wood.
Build Coats
At this point you could have a piece with some filler and sealer, perhaps a little color, but not much finish to protect the surface.
One of the classic finishes is shellac as a final coat, not just a sealer. However, shellac is totally soluble with alcohol (even beverages), and quite subject to water damage. I do not recommend shellac as a modern final finish (although it is beautiful in a museum).
Lacquer is a traditional furniture finish, so is my primary choice. Multiple coats of solvent-based lacquer "burn in" and become a single coat, which simplifies the application - it doesn't matter whether each coat is thick or thin. Lacquer is normally sprayed since it dries so fast that it is hard to brush without leaving brush strokes. Finger nail polish is lacquer, so fingernail polish remover will remove lacquer, as will Deet insect repellant and some other household chemicals. Catalyzed lacquer goes through a hardening process in the weeks after it is dry, making it more durable. But Target Coatings has a water based lacquer with full burn in. (I have not found any other vendors whose water based lacquer has burn-in. If you have to sand between coats, it does not burn in!) And the Target EM6000 Acrylic Lacquer is comparable to the Catalyzed lacquers in testing by the Kitchen Cabinet Manufacturers Association - KCMA. It was the lacquer that led me to the Target Coatings products.
I like to spray 3 to 8 "build" coats of finish. I use gloss, with no additives to dull the finish, for all the build coats - and to simplify the spraying since the additives settle in the gun, as they did in the paint can. If the coats are thin, 8 is not a lot (and they dry so fast they can be applied every 10-15 minutes). But if you are getting thick, even coats, a couple coats is fine, and 8 coats is too much and will leave your surface with a "plastic" appearance. This may be desirable on the "grand piano" finish, or on a decorative turning, but probably will not look good on most furniture.
Note that we started with a perfectly smooth surface after the sanding sealer. By the time several build coats are applied, the surface may not be as perfect - there may be the occasional speck of dust, drool, pollen (if you are spraying outside) or other sin. Therefore, before I do the final coat, I often sand with 400 grit until the surface is absolutely perfect again.
Are we done yet?
If the goal is a lacquer finish, the final coat could be one more coat of lacquer, or you could take the sanded lacquer, and rub it out to the final finish (see below). However, on work surfaces like desktops or a dining room table, I like an even harder finish than lacquer. A conversion varnish is harder (scratch resistant) and more chemical resistant than a lacquer, in part because the varnish cures by a chemical reaction rather than just the evaporation of the solvent.
The good news (to me) is that Target Coatings also has a conversion varnish, EM8000cv, that sprays and cleans up much like their other finishes. The bad news is that it is not a lacquer. Each coat of finish dries as a separate coat, like ordinary paint and varnish. That means that the final coat has to be sprayed perfectly - no burn in between coats to combine thin areas or other spraying flaws. If I am finishing with a varnish on top of something else, I normally apply two coats of varnish.
Is there something even harder than varnish? In an solvent based finish, that would be polyurethane. Many people have had good luck with poly on furniture, but I have not. Poly is fussy about surface preparation, but when it has cured is very hard and scratch resistant (I love it for floors) but that hardness makes it brittle as well. Years ago I coated my eat-in-kitchen table with polyurethane, and my son and his friends managed to get several divots in the finish. Of course he has no idea how it happened, but the divots arrived during the period that the table was the place for school homework (and playing with hot wheels). (Target does make a water based polyurethane, EM9000, but I have not used it. My bias against polyurethane on furniture is too strong.)
Target does have a finish even harder than polyurethane - EM9300 polycarbonate urethane. I used it on the service counter in an auto repair shop, and after several years of customers sliding car keys across the counter, it is hardly showing any wear. I have used it on end tables where coasters are not routinely used, and it has withstood abuse for over a year.
Finish the finish
Some people like the gloss that comes from a sprayed finish, just as it comes from the gun. If that is your goal, sand the build coats at 400 grit or finer until it is perfectly smooth, then spray the final coat. In this case you may wish to use satin or semi-gloss lacquer for these last coats.
If there are imperfections in the finish - the speck of dust or pollen, the bug track, they need to be sanded with flat sandpaper - sponges or synthetic steel wool will not flatten the surface. If you want to soften the gloss on a wood like oak, with the deep grain, you need something that gets into the grain, so that the valleys are not shiny. If all I want to do is get "in the cracks of the grain" I use a white 3M pad. If I want to soften the gloss as well as get into the grain, I use a rougher synthetic "steel" wool pad, such as Mirka's Mirlon red (fine)or dark gray (ultra fine).
My favorite finish is to sand the final coat at 400 grit or finer until it is perfectly smooth, then increase the grit to increase the shine as far as desired. 800 grit will give a very nice semi-gloss. By the time I am at 4000 grit, the finish is near gloss - take a soft rag (like a towel) and rub the finish to the final shine. (You can use an ROS with the towel to finish the rubbing.)
I no longer routinely use furniture wax as the final finish. Wax is less durable than the finish, so adds work to do it and work to maintain it.
If you are really looking for that grand piano finish, you need to go finer than 4000 grit. I find it easier to start using Menzerna rubbing compounds after sanding through 800 grit. See my page on rubbing the finish for details on the rubbing compounds.
I find I can "get away with" dry sanding as soon as the finish is "dry," even though it is still quite soft (an advantage when sanding and rubbing). Wet sanding and rubbing compounds require the finish to be cured - far more than just dry. Most of the Target coatings are dry in 10-20 minutes, but take up to 5 days at 70 degrees to cure.
If you are planning on a high gloss "grand piano" finish, you need to use a wood grain filler on open grain woods like oak, mahogany, walnut, and cherry. If you want the grain texture to show, skip this step. The one I use is at the left, but I am sure others would be fine. This is typically a paste-like substance that fills the pores of the wood (and minor sins like imperfect glue joints). It is thicker than latex paint, and weighs a LOT more - it actually fills the grain with a fine mineral. Slop some on the surface and rub it across the grain with a plastic putty knife. Since it is water based, it dries very quickly (smear fast). Once it is dry, sand down to the surface of the wood. As a mineral, it doesn't absorb dye, but it can be stained - however, I find the brown filler works on most wood and doesn't need extra color. The "neutral" is almost white, which creates an interesting effect, but less natural (to my eye) than a darker filler. Some people argue that the grain filler should be used after sanding sealer and dye, but I prefer using it before, so I can sand like crazy without worrying about removing some of the color. 
There are two purposes for sanding sealer. 1) to fill the pores (including those left by the grain filler, if any). And 2) to bring out (pop) the color of the wood. For years I couldn't believe you could or should use a sealer before a stain or dye (in this role sometimes called a conditioner). A thin coat (diluted 50%) of sealer lets the part of the wood that would soak up too much stain (dark and blotchy) satisfy the thirst. It then can be lightly sanded (if you put too much on) so the surface of the wood can absorb stain evenly. I have finally become a believer.