Shop Tour VI ~ Finishing

Shop Tour / Guitar Building Process – Finishing

You might think that once the woodwork is done, the instrument is almost done. Alas, you would be wrong. A proper finish can take a great deal of energy, effort and time. I have worked with many different finishes, nitrocellulose lacquer, polyurethane, polyester, and various oil finishes. Here I will be showing how I work with nitrocellulose lacquer and polymerized tung oil (Tru-Oil), as well as 2 methods of staining wood.

Let’s start with staining. The Meridian semi hollow is on the bench for finishing. The client wants a darker, aged look to the mahogany and a subtle Tru-oil finish. In my opinion, the most effective method of aging certain woods(wood possessing tannin, i.e. mahogany) is a potassium dichromate (PDC) solution. PDC comes in a crystalline powder form. The powder is mixed into distilled water and stirred until fully dissolved. More powder is slowly added until the water cannot absorb any more powder and crystals start collecting on the bottom. This is considered a 100% solution. A 100% solution is very strong and will make a very dark finish, too dark in this case.

Every piece of wood has different tannin content and tests need to be done on scraps from every project to achieve the desired effect. Tests indicated that a 12% solution was ideal in this case. Below is a photo of PDC in powder form and in various strengths of solution. It should be noted that this is some seriously nasty stuff and should be handled and used with great care. I wear a mask and gloves when mixing and applying the stuff.

Below is the guitar after finish sanding. Because the PDC is suspended in a water solution it will raise the grain. This would need to be sanded down and that could cause uneven coloration due to sand through potential in some vulnerable areas. To avoid this I raise the grain previous to the application of the PDC solution. Another and perhaps more important reason for raising the grain previous to the application of the PDC solution is to avoid the PDC becoming airborne in the very fine dust created when sanding. Again, this is nasty stuff and this would be the most hazardous form. I gently wet the wood, let it dry, sand back the raised grain and repeat until the grain no longer raises when it is wetted. In this case, 3 gentle wettings did the trick. I finish sand down to 600 grit paper in order to get very consistent saturation and avoid blotching or streaks. Also, this guitar will be gating a Tru-oil finish. Oil finishes require sanding to higher grits because they have very little filling capability and even very light scratches will show.

Here it is after PDC has been applied. Although a 12.5% solution sounds weak, it actually packs a bit of punch. In order to avoid lap marks and possible uneven coloration, I did two 6.25% solution applications which gives the same effect as a single 12.5% solution. After the second application evaporated, I gave the whole guitar a distilled water bath followed by a naphtha bath to remove any residual PDC left on the surface. I never see anything on the rag after the baths, but I do it anyway. It’s a cheap way to get valuable piece of mind.this guitar is ready for Tru-Oil.

Sunburst finishes. For sunbursts I use TransTint concentrated dyes directly to bare wood. By using these dyes I have the ability of mixing custom colors and applying these colors in differing strengths. I use the dyes suspended in water. Like the potassium dichromate solution, I have to raise the grain before applying the stain. I apply the stain with cotton balls wrapped in squares of t-shirt rags. On this guitar I used honey amber as the base coat, then blended orange to red mahogany to achieve the sunburst. I then toned it down a bit with a weak solution of dark vintage maple on the whole thing. I didn’t take pictures as I was applying because I find the colors blend much better if you do it all in one session without letting any of the colors set before the next one goes on. Here it is. This one would probably be referred to as a teaburst.

After the glue-up, from a few angles. This guitar has a false maple binding. This is done by masking the body, scoring the binding edge and sealing with finish before applying the stain.

With this guitar the idea was to play off the dark rosewood, so I went with a dark tobacco burst. I used honey amber, vintage maple, red mahogany, medium brown and dark walnut on this one.

With the neck glued on…

The Meridian semi hollow body is getting a Tru-Oil finish with no pore filler. Tru-Oil is a hand applied finish. With this type of application the finish is applied only a few drops at a time over only a few square inches at a time, overlapping as you go. The finish is rubbed on and almost immediately rubbed off, leaving minuscule amounts of finish on the instrument. After repeated applications the finish builds up to a protective coating. The more coats, the shinier the sheen. Compared to other finishes, this lacks slightly in durability, however, in my opinion, this is the thinnest and most tonally transparent finish one could apply. Here it is after 1 coat of Tru-oil.

After 6 coats…very thin coats…

…and the top…

As of now the guitar is sealed and protected. Now I will continue applying micro thin coats until the desired sheen is achieved. Maybe 10,maybe 20 more,??? Here it is after…I lost count at 14, probably 20 coats.

The client found the sheen was a little too shiny. I probably should have stopped at about 10 coats…oh well. Not to worry though, there is a quick fix. I knocked the sheen back with OOOO steel wool and applied a shop formulated carnuba/beeswax recipe.

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This Contemporary will also be getting a Tru-Oil finish, however I will be using a different method of application. Here I will be doing a slightly thicker finish by creating a sanding slurry with the Tru-Oil in order to fill the grain slightly. The body wood on this guitar is Spanish cedar, which is a very porous wood. No pore filler will be used, so I don’t expect to completely fill the pores, but I will get them mostly filled. The first several coats are applied with fine grit sand paper creating the slurry, filling the grain. Subsequent coats are vigorously applied with fine linen in overlapping patches until the finish stiffens up and is burnished by the linen. This is a very time consuming finish, but it sure has a deep look and, like any Tru-Oil finish, feels silky smooth in hand. After just a few coats…

…a few more…

…and more…

And done! I will wax this one as well. Wax can be periodically applied as need or desired.

Most clients request nitrocellulose lacquer. It’s tried and true and that’s why it’s a standard for Certified 7’s. The Meridian solid body will also get nitro. Very thin, durable finishes can be achieved with conscientious application. After sanding to 320 grit a sealer coat is applied.

Then pore filler is used to level the surfaces. No photos of this. Just picture smearing a gooey paste all over your finely prepared instrument, letting it cake up and dry, then sand the thing down with 320 grit sandpaper all over again. Sounds screwy, and it’s no fun, but it’s gotta be done in order to achieve a high gloss finish. Once the surfaces have been filled and leveled, I start spraying 50% reduced nitro. After each coat, imperfections are sanded out and recoated. Here they are after 3 coats.

I level sand so that the surface is perfectly smooth without breaking through the finish.

3 more coats and let them cure for a month or so.

Final sanding and buffing the finish. Here is a shot of the Certified 7 teaburst. Final coats have cured and are ready for level sanding and buffing. In this photo only the back has been sanded.

The entire guitar has been meticulously sanded to between 800 and 1200 grit. I sand the flat surfaces down to 1200 grit and the curved surfaces to 800 grit. The big, wide flat surfaces are more difficult to buff so I sand them to a higher grit.

Here is the buffer. Pretty standard buffer. I use the wider buff on the right for the majority of the guitar and the thinner buff for tight areas(mostly at the neck joint).