This is the Holy Grail for many woodworkers. Or at least you'd think so by how tight lipped woodworkers can be about their finishing techniques. "What finish did you use?". For some woodworkers you may as well have called their Mothers ugly by the look you may receive.
Finishing wood is an art unto itself. With that being said though, it's still not as complicated as some would have you believe. Much of the confusion comes from the manufactures themselves, mostly in the form of how they label products.
Why do so many woodworkers simply throw their hands up in the air and just use one type of finish for all their projects? Because they're afraid of trying something NEW!
Well, as with most things that seem complicated, if you take the time to break the process down into smaller pieces, it becomes much easier to understand. With this understanding, the decision making process becomes much easier.
When choosing a finish there are six major qualities to consider:
1) Appearance
2) Ease of Application
3) Protection
4) Durability
5) Health Concerns
6) Ease of Repairing
When deciding on a finish for it's appearance there are three basic questions to ask yourself:
1) Will this finish build up on the piece?
2) How Transparent do I need the finish to be?
3) Will the finish add any color to the wood?
Finish build up:
If you want a finish to build up on wood you must us a film finish. For our discussion here, we'll only be looking at finish that can be applied by hand (not sprayed on). With that in mind our choices would include: Shellac, brush on lacquer, varnish and water base. Oils don't cure hard so they have to be applied in very thin coats (with the excess wiped away) which doesn't allow them to build up on a surface.
Transparency:
This is probably most important when you're dealing with a light colored wood such as maple. De-waxed shellac, lacquer, and alkyd varnish (if the can simply says "Varnish", it's probably is made up of alkyd resins), are the most transparent finishes. Natural shellac, oil-based polyurethane and water base are the least transparent finishes.
Color:
Finishes that contain oil (including varnish) will yellow with age. This can be desirable on darker woods adding "warmth" to the look of the wood. However, on light colored woods it may be undesirable. Wax and water base finishes add very little "warmth" to wood. Lacquer and clear/blond shellac add a small amount of yellowing but not to the degree of oil based finishes. Garnett and button shellac (or lac) colors add a deep orange/brown color that is nice on darker woods and can give an antique appearance. Orange shellac colors the wood purple. Just kidding. The orange color of orange shellac adds a lot of "warmth" to wood, especially kiln dried walnut.
Now let's look at protection and durability of finishes.
Protection:
Most wood looks nice completely unfinished. So why do we even bother to go through all the steps necessary to finish a piece of furniture? Two reasons are: To protect the wood and add to it's durability.
The best reason to protect wood is to slow down the water transfer between the wood and it's environment. Wood movement is caused by moisture transfer. When wood has a high moisture content it swells, mostly across the grain. The opposite is true when wood loses moisture. This movement can play havoc on glue joints.
For a finish to be resistant to water and water vapor a finish must be fairly thick. The best finishes to use for this are varnishes and water based (there are others but they must be sprayed on). Shellac does a good job against water vapor but a poor job against direct water contact. Wax and oil finishes do the poorest job against water and water vapor mostly due to their thin coats.
No finish will completely stop the transfer of moisture between wood and it's environment, some just do a better job of slowing it down.
Durability:
The durability of a finish must take into consideration wear resistance, solvent and other chemical resistance, and heat resistance.
Varnish and water base finishes do a good job with wear resistance. Shellac and brushed on lacquer do a fair job. For finishes that can be applied by hand, varnish is the only one that gives good chemical and heat resistance.
Ease of Application:
One overriding factor that must be taken into consideration here is: How fast does the finish cure?
If a finish cures fast we can put more coats on faster. The downside is that the finish can cure too fast causing it to smear if we drag our brush or rag through it again. If a finish cures slowly we can cover large areas without fear that it will start to get sticky, which is great for large pieces. The downside is you must work in a dust free environment due to the possibility of dust settling on the finish before it cures. You can get around this somewhat by wiping off the excess finish so that it will cure faster, but many more coats must be applied.
The easiest finishes to apply by hand are: Oil, oil blends, wiping varnish (thinned varnish), and gel varnish.
Oils and oil/varnishes cure slowly, as does varnish. Water base dries faster than varnish but not by much. Shellac cures fast and brushed on lacquer cures slow enough to be able to brush on.
Health Concerns:
Finishes that can be applied by hand that cause the least health problems are linseed and tung oil, water base and shellac. Linseed and tung oil contain no solvents and water based finishes contains only a small amount. The solvent for shellac, denatured alcohol, is relatively safe unless its ingested or breathed for a long period of time.
All the finishes we've talked about are combustible except water based so avoid using them around flames or sparks.
All finishes have some degree of odor to them. It's never a good idea to breathe in solvents so one of two things should be done. Use the finish outdoors, or use a respirator that is approved for use with the finish you're using.
One more point about the safety of finishes. All finishes are safe to be in contact with food after they have cured completely. This doesn't come from me but from the FDA. All the metallic additives used are safe once they have cured. This is due to lead no longer being used in finishes since the 1970's.
Ease of Repairing:
One rule of thumb to keep in mind: The thinner the finish, the easier it will probably be to repair. Wax, oil, oil/varnish, shellac and brush on lacquer are all fairly easy to repair. Varnish and water base are the hardest to repair.
Armed with this information, you should be confident in choosing the right finish for your next project.
About the author: Craig Stevens has been a furniture maker for over 10 years who enjoys teaching others the joys of learning woodworking. If you are interested in learning more about woodworking, or in teaching your kids woodworking, go to our website to find tons of free information.
http://www.WoodworkersResource.com
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