An Overview of Wood Finishing
by Bill Wilkat, Jan. 2009 Edited Aug. 14, 2018
Oh man, there’s so much more to this subject than I will ever be able to cover in one article, but I can perhaps help you get started on the path to producing a really nice finish with a few words of advice based upon my experience and my trials and errors—yes, trials and errors. I stress this latter point because I know of no aspect of woodworking that has a greater learning curve, and the only true to way to get from point A to B is through one’s own trials and errors. If this does not sound too encouraging, please understand that I’m only trying to prepare you for what’s ahead—a lot of work!
Yeah, “work” is a “four” letter word, but not in the vulgar sense, but simply to stress that “you don’t get nothing for nothing” you need to work for it! And so, now that you are ready to say “screw this!”, those of you who are willing to put in the effort, stay with me, and the rest of you—well, click on out of here–and hire somebody to do it for you! LOL!
I can honestly say that finishing wood can be among the most challenging, but also one of the most rewarding tasks of any woodworking project, and musical instruments rank highly in that regard. The first, and absolutely the best advice that I will give you, is to practice on scrap pieces of wood, or old guitar bodies that you don’t mind sacrificing, or that you wont shed a tear for, should you botch the job. There’s nothing worse than tackling the finish on your pride and joy and ruining it by doing a sub-standard finishing job on that glorious hunk of wood that was destined to become your baby—the one you were planning to play with all your heart and soul, and proudly, (perhaps even smugly), display to your family and friends.
That being said, the best place to start is by reading everything you can get your hands on about wood finishing, and that means trips to the library, purchasing books on the subject, and scouring the internet for articles like this one! Like many of you, I have been a fan of magazines on playing bass and guitar, and this is how I first learned about many of the master luthiers and a number of them have published books that cover everything about instrument building, finishing and repair. There’s no shortcut here, and I must say: read, read, and re-read—and only then will you be prepared to begin the real learning, by applying what you’ve read, bit by bit.
Virtually everybody loves the wet look of a mirror like lacquer finish that you feel you can dive into, but be advised that it is not always the best choice, and if applied too thick, can alter the tone of your instrument—however, it’s not always a negative thing, and personal taste is too subjective involving lots of emotion, so I’m not going to go there. What I will tell you is that there are many ways to finish wood, and which ever way you decide to go, a lot of the preparation is similar, and the first step involves doing a great deal of sanding once you’ve shaped and curved body and neck to your liking.
Thus, let’s look at how to proceed for this first step. Most of you will undoubtedly be using solid body construction, but hollow, (or semi-hollow), wood bodies need the same attention to detail, and it begins by using various grades of sand paper, and gradually working you way up to finer grits until you are ready to apply your selected media. For most of us, that means starting with a #150 coarse grit and working up to #220 fine grit, but I have and often gone beyond that to #320 fine grit before calling it quits. Again, nothing is absolute, but you don’t need to go crazy here, you just need to ensure that the surface is smooth, free of imperfections and tool marks, and clean, and often #220 is adequate. This is not a something that is done quickly—it takes time and lots of elbow grease, and patience. Once you think you are finished, you probably are not!
I say this, because even under the best of light conditions, and even with seemingly meticulous scrutiny, we often discover later that we could have done it better, or missed a spot, or much to our surprise after the initial finish is applied, scratches are revealed that went previously unseen. Remember what I said about patience and elbow grease?
During the sanding sequence, it’s also vitally important to raise the grain of the wood and this requires introducing moisture into the process—not by wet sanding, but by wiping down the wood with water, using a damp rag (not dripping wet), and allowing the wood to dry. Once dry, the grain has been raised, the grit of the sandpaper can do it’s job and shear off the fine fibres that protrude above the surface, and make the wood feel smooth to the touch. Normally this must be done towards the end of the sanding process, and often needs to be repeated more than once. Also, final sanding should always be done by hand—avoid power sanders since you can not “feel” what you are doing, and they cannot properly reach into all the curves or gently follow the radii as required.
Make a Book of Your Project
To do final inspection of the surface, it is best to use Naphtha to wipe down the surfaces as it will not raise the grain further once you have gained a nice smooth surface, but it will reveal imperfections that require further attention. Tests will have to be done with scrap pieces of wood as well to test both stain colours and finishes applied over them.
Next, many types of wood—particularly species with open grain, will require a wood filler, and there are products available to assist with this task. If you don’t use them, it will take a lot longer to fill the surfaces of the wood to obtain a level surface, that will be the foundation of your finish. And, levelling the finish is the process of obtaining a smooth by cutting off the peaks (or high points) of the surface, in preparation for the next application of the finish media. This actually applies to all finishing media, but is most critical for high gloss finishes whether it is nitro-cellulose, or polyurethane, etc.
Sealers are often the next step, and are required to avoid having the finish drink into the wood which can result in considerable grief if you skip this step. Again purchase the correct products for this step, and you will be rewarded. However, before applying any sort of sealer, bear in mind that certain types of wood are oily in nature and require special preparation to reduce excess oil and minerals that can interfere with the applied finish. I’ve had good results by wiping and cleaning these woods with Acetone, which seems to do an excellent job, and follow this with Naphtha, since it never leaves behind a film.
I favour oil finishes and highly recommend them, especially for those who don’t have spray equipment, or for individuals (like me), who are sensitive to the fumes exuded by lacquers and/or synthetic finishing materials. In addition, an oil finish can be completed in less time than most high gloss finishes, and do not require long cure or drying times, which many people do not have the patience for. Today, many factories get around these problems by having costly spray rooms with special ventilation equipment, and cure time can be reduced using ultraviolet chambers designed to speed up the process. For the beginner, these are out of the question, and we need to establish a schedule to follow, and if we follow it diligently, we will get the results we are seeking.
Application of high gloss finishes for the beginner can be done using, a brush, spray cans, or a spray gun and a compressor but know what you are getting into before you begin, and your chances of success will be greatly improved. Again, research involving lots of reading makes the difference. If you are lucky enough to have a mentor who can show you first hand, even better! I can only tell you that if you try to rush any aspect of finishing, it will usually backfire on you, so don’t say I did not warn you.
An important tip that I will share, is to never do more than 3 spray applications per day, and to let these cure and dry for up to 2 weeks before doing any sanding, and then you can apply the next coats. This is the process of creating a “build”, which can be levelled through the sanding process. You’ll know when you’ve reached the final stage, but once again, you will need to use various grits of sandpaper before you are ready to consider buffing out the finish. This will involve the technique of “wet sanding” and it is normally done with water, and the sandpaper is soaked and frequently rinsed in the water. The addition of a little bit of liquid soap into the water is done by some, but I prefer to avoid this, and instead, I change the water as soon as it tends to get cloudy. I normally start with a #400 or 600 grit but before long I am using #800. 1000 or 1500 grit. It’s important not too use a grit that is too aggressive since “rub-through” down to the wood is very easy to do, and something that is definitely to be avoided whenever possible—in particular if you are doing a clear high gloss finish over a stain.
Once you’ve reached your final level of wet sanding, and the surface has a beautiful even dull patina all over, you are ready for buffing. I find that often I did not need to exceed a #1500 grit, but the finer you go, the better the end result can become. Mind you, the rubbing compound takes over the work of fine sand paper, and a medium compound, followed by a fine compound usually gets the job done. Of course, a buffing wheel yields the best result at this stage, but hand buffing using a drill and a buffing pad can be substituted. Either way care must be taken to avoid “rub-through” which can happen in a heartbeat, as the buffing pads and/or wheels generate heat quickly if held too long in one place!
The products available today are fantastic, and many builders are now employing the latest products with amazing results. Once upon a time, only nitro-cellulose was the primary lacquer of choice, but today modern products like polyurethane and polyester are preferred and are tough and durable. Do-it-yourself types can also opt for water based finishes and great results are possible even with brush-on and wipe on products, so don’t shy away from them—test them and have fun experimenting. Always follow the manufacturers recommendations and safety precautions as well.
In the future, I’ll elaborate on my personal method of doing an oil finish, but for now, I urge readers to do the smart thing and read some of the superb publications already on the market. Remember, use your local library if you can’t afford to buy them, and read, read, read, and practice, practice, practice!