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The goals of wood finishing are to enhance the appearance of the wood, and to protect the wood adequately for its intended environment. The products sold for these purposes are legion, and I will leave it to another page to decipher the hype and separate the numerous products into orderly groups. My intent here is to provide a survey of the finishing process as a whole and to touch on the major products and procedures that are used to achieve a fine finish, as well as giving some general pointers to help someone who is new to finishing to get started in the right direction.
My most common finishing advice is "the product you are most comfortable with is usually the best product for your job." Interior trimwork, for example, is not usually subjected to heavy use or abuse, so any interior, medium-duty or tougher finish will work fine. The secret to a great wood finish is, ultimately, practice. You use a product until you know it inside and out and achieve consistently good results with it. The only time to switch products is when your favorite product happens to be ill suited for the job, or when you have mastered it and want to move on and try something new.
I work in a woodworking retail store, and most of the customers who have trouble with finishing in general have never consistently used one product, but rather they've jumped from one to another finish for everything they've made, depending on what's hot in the magazines and on the online forums at the time. No wonder they can't understand finishing - they've never used anything long enough to establish a knowledge base, so everything they learn blends into a confusing mess! It also doesn't help that the manufacturers of most wood finishes label their products as a "black box," never really explaining the basic components or their benefits to the user. Most products simply say something to the effect of "gives a beautiful, durable finish." Yeah, thanks for the information guys, but that's the same as saying "this is a can of mysterious chemicals that you apply to the wood to make it look nice." It tells me nothing about what's in the can or how it will actually affect my project.
To learn about wood finishing for myself, I began by reading several books dedicated to the subject, and finishing 8" square samples of woods and plywood with various products (and re-finishing as necessary, until I got it right). I also began working on the beginnings of the material presented below and eslewhere on this site relating to wood finishing. In researching this material, and in college chemistry class, I learned a lot about how the components of common finishes work and what they do for the final product. Finally, teaching classes and helping customers with finishing problems helped to cement this information in my mind, and the practice I gained helped me learn to organize it in a way that is, I hope, a bit easier to understand than the way in which I originally learned it.
Outlined below is is a general sequence of operations that are performed in order to consistently achieve a good finish. Not every step listed here must be done; in fact, it is rare that every step should be followed in a single finish, but the idea here is to provide a complete list of operations and the order in which they should be done. This is meant to be merely a starting point in understanding the finishing process as a whole. You'll also want to review the pages on dyes, stains, sealers, grain fillers, and clear finishes in order to know which of these products you might want to use on any given project and the details of what they do and how they work.
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Careful surface preparation is the first and most important step in producing a professional looking finish. While it is possible to achieve a very fine surface using hand planes and scrapers, sanding is usually a much simpler method. Start with 120 grit sandpaper on fairly smooth surfaces, or 60 grit for rough or torn surfaces. If you are refinishing after stripping a piece, only minimal sanding should be required and you can usually start with 150 or 220 grit depending on the quality of the surface. An aluminum oxide abrasive works best, and a random-orbital sander can make quick work of large, flat surfaces, although any power finishing sander or hand sanding will work. When sanding by hand, always use a sanding block, as this will prolong the life of your sandpaper, leave a finer scratch pattern, and is more comfortable over long periods. Sand with your first grit until all imperfections have been removed and you have an even scratch pattern over the entire surface, then move on to the next finer grit. At this stage, sanding in an orbital pattern or diagonal to the grain will help to speed up the cutting action of the sandpaper. Progress through each grit of abrasive until you reach 150 grit. You can tell when you have sanded an area sufficiently by observing the degree of scratching left by the sandpaper. This is best done by working in a semi-darkened room and placing a light such that the rays of light hit the surface at a shallow angle, highlighting high and low spots and scratches. Another method is to scribble lightly over the entire surface with a soft pencil, then sand until all pencil marks are completely erased. After sanding with 150 grit, switch to sanding parallel to the grain, or use a reciprocating (inline) finishing sander, if available. Continue progressing through sanding grits to at least 220.
At this stage, how much additional sanding you do depends on the wood and the finish to be used. Oil finishes and clear satin varnish tend to hide fine scratches and dips in flat surfaces, while stains and high gloss finishes such as lacquer tend to highlight them. Finer grained woods and dense woods like ebony will need to be sanded with finer grits, while softer woods like pine will not really benefit from sanding with grits finer than 320. For most clear finishes, it is sufficient to finish up by sanding by hand with a sanding block, using 220 grit paper and sanding only parallel to the wood grain. When using any stain, dye, or tinted finish other than paint, finish up by sanding by hand and only parallel to the wood grain. It is extremely important to leave a consistent, fine scratch pattern if a stain is to be used, since stains tend to highlight cross-grain scratches, so be meticulous and change to fresh sandpaper as soon as a sheet stops producing fresh sanding dust. Once you have finished sanding, it is time to clean up as much dust around the shop as possible, to prevent it from becoming a problem later on.
Up until now, it hasn't made much difference what you are finishing or what type of finish you plan to use. Here is where you need to make the final decision about what you want your finished project to look like. Dyes and stains can make plain wood look exotic, or a new project look centuries old. Grain fillers can be used to get a glassy smooth surface on coarse grained woods for a finer, more formal finish. Glazes can add dimension to carved or textured surfaces, and an antique effect to pieces of any age. Finally, different topcoats can enhance the natural color of the wood while providing varying degrees of protection from abrasion, moisture, fading, and dirt. The following list gives the appropriate order in which to use all the different finishing products. Which of them you use on your project is entirely up to your own taste. If time is a factor, keep in mind that the more steps you use, the more time and work will be involved. Beware of hidden time factors such as the actual time it takes for a finish to dry completely given the current weather conditions.
Finally, here's one final bit of important advice: ALWAYS test a new stain and/or finish on scrap wood that has been prepared the same way your project has! This lets you spot all those nasty little problems before they become big refinishing jobs. Things like blotching on cherry and topcoat / stain conflicts will only show up if you test the finish first.
Having spent a few years building my own finishing skills and helping others with their own finishing difficulties, I've learned a few bits of advice that can go a long way toward achieving finishing success.
ALWAYS test on scraps first! Some products just don't get along with each other. Finishing is chemistry, and you can't always spot problems until the chemicals actually get together on a piece of actual wood. If you are refinishing or there are no scraps available, test on a hidden or inconspicuous area such as the inside of a drawer front, bottom of a table top, etc.
Take your time. When people "ooh" and "aah" at a piece of furniture, it's the finish they're looking at; so take the time to do a good job on your finish. It should take about as long to properly finish a piece of furniture as it took to build it in the first place, so don't try the old "slap on a couple coats of poly the night before the deadline" trick. If you cheese out on the finish, it will show. Good preparation is the biggest key to a good finish, so especially take your time to do a good job of sanding and other preparation work.
Even a great finish can't fix lousy construction. If you built it wrong, wood filler is not the answer. Take the time to re-build parts until you've got something that is worth spending the time to put a good finish on it. Trust me, it's worth it if you plan on keeping the thing around for very long.
Find a finish you like, and stick with it until you've got it down (i.e. practice makes perfect). You'll never learn the subtle differences between one brand and another or one type of finish and another if you don't use one until you've learned it forwards and backwards. There's absolutely NOTHING WRONG with finishing everything you make with waterborne polyurethane. You'll learn that you can make it look like a hand-rubbed oil finish, or like a high-gloss lacquer, or whatever, depending on how you color the wood, how you apply the polyurethane,and what you do to it after the finish has cured (i.e., rubbing out).
Take care of your tools. Finishing tools come in cheap, disposable varieties as well as more expensive sorts. Better equipment does tend to give better results with film-forming finishes, so invest in your equipment and take the time to maintain it. A good brush should last for years, and cleanup isn't as big a hassle as you'd think.
Hand Applied Finishes, Jeff Jewitt, Taunton Press, 1997
The New Wood Finishing Book, Michael Dresdner, Taunton Press, 1999