A general technique for gluing the steel string guitar bridge for instruments built one at a time is to finish the instrument without the bridge attached and then attach the bridge. This requires scraping off the finish from the top at the bridge location, a process which puts a sharp scraper in close proximity to the nicely finished top. I am a person for whom any screw up possible is a screw up accomplished, and so this scraping step always makes me a little nervous. Recently I’ve been making use of a production technique, using a bridge mask applied to the top before finishing. The mask is removed prior to gluing the bridge on, a process which for me is a whole lot less likely to result in a gouge in the top, and takes less time than scraping off the finish. This article originally appeared in American Lutherie.
Initial appearance: November 07, 2021
Last updated: November 07, 2021
Copyright © 2011 R.M. Mottola
[This article originally appeared in American Lutherie #108.]
A general technique for gluing the steel string guitar bridge for instruments built one at a time is to finish the instrument without the bridge attached and then attach the bridge. This requires scraping off the finish from the top at the bridge location, a process which puts a sharp scraper in close proximity to the nicely finished top. I am a person for whom any screw up possible is a screw up accomplished, and so this scraping step always makes me a little nervous. Recently I’ve been making use of a production technique, using a bridge mask applied to the top before finishing. The mask is removed prior to gluing the bridge on, a process which for me is a whole lot less likely to result in a gouge in the top, and takes less time than scraping off the finish.
The bridge mask can be fashioned from any kind of heavy paper that can be cut to the outline of the bridge. I use big white paper peel and stick mailing labels (Avery #8126) which come two labels per sheet. Making the mask is quick and easy, more so because I use CAD software to draw my instrument plans and I make the bridge mask drawing at the same time as I make the bridge drawing. But the process is simple enough to do with a pencil and either an outline template for the bridge or the actual bridge. First the CAD description.
The outline of the bridge mask is just an offset from the outline of the bridge itself. I generally make the mask about 0.02’’ smaller all around. The instrument centerline is copied onto the mask as well. Then the fingerboard side lines are extended so they end somewhere in the bridge mask. The distance from the forward edge of the nut to the top edge of the bridge mask along those lines is determined, and that value is printed on the mask. I also include text identifying the instrument for which the mask is intended. If you fix your bridges in position using locating pins or some such, you can also print the pin locations right on the bridge mask. A bridge mask for a classical guitar built using dimensions in US customary units is shown in fig. 1. Although classical guitars generally have their bridges glued to the top before the instrument is finished, I find it easier to polish the top without the bridge on.
The bridge mask is printed onto the mailing label material for use. I like to use a very light gray color for the lines and text on the bridge mask, because black tends to dissolve in lacquer and shellac and make a mess of things during finishing. The big labels I use can accommodate more than one copy per label. If you don’t have a calibrated printer, it is a good idea to print the bridge masks on the same printer and with the same paper orientation that you used (or will use) to print out the outline of the bridge. That way any dimensional errors will be the same in both the mask and the bridge outline itself.
It is not a big deal to make a bridge mask directly on a paper plan, using the same steps described for a CAD drawing. It is also possible to make one using the guitar itself as long as its neck and fingerboard are attached. Draw a centerline down the middle of a paper label of appropriate size. Don’t take the backing off yet. Position the label on the guitar top approximately at the bridge position, aligning the body centerline with the centerline drawn on the label, and tape the label to the guitar top with masking tape. Now go through the usual motions of locating the bridge (or a template of the bridge), which should end up on top of the label. Hold the bridge or template in position and then draw all around it with a pencil onto the label. The bridge mask should end up a little smaller all around than the outline of the bridge. It is easy enough to draw this offset outline for a rectangular classical bridge, but it could be tough for a bridge with a real curvy outline. In the latter case, just draw in the offset for the top edge. Then with a ruler held along the side of the fingerboard and butt up against the nut, draw the little extension lines on the mask and also write down the distance from the nut to the top edge of the mask along this line. Repeat this last step on the other side of the fingerboard. The mask is ready to apply and can be un-taped from the top.
The mask can be cut out using scissors and/or hobby knife. Since I generally make one-of-a-kind instruments I find it is a good idea to have the bridge already made before applying the mask to the top. That way the mask can be checked against the actual bridge to be sure it is just a bit smaller all around than the bridge is. If it is not, the mask can be trimmed as necessary.
If you use pins to locate your bridge and if you included the pin locations on the mask, you can use these to locate the bridge mask. If not, the mask can be located by measuring down the sides of the fingerboard, from the forward edge of the nut (photo 1). Draw a small pencil mark on the top at the distance noted on the bridge mask, plus just a bit (a pencil line’s width) more, as shown in photo 2. Repeat this step for the other side of the fingerboard.
Now a dry run of locating the mask is done. The mask is aligned with the centerline of the top and moved so the two extension lines from the fingerboard edge on the mask meet the two pencil marks just drawn. If your neck and fingerboard alignment are perfect everything will line up perfectly. But this is not often the case, so the reason to do the dry run is to note how you will need to make adjustments to the lateral position of the mask. In general the adjustments involve moving the mask slightly off the centerline so that the extension lines and the penciled location marks match up. If you do this, it may help to pencil in the new bridge centerline on the bridge mask.
Once the dry run is done it is time to stick the mask down to the top. The backing is removed from the mask and it is applied to the top, aligned as in the dry run. Be sure to place it down so the top edge of the mask just covers up the location pencil marks you drew on the top (photo 3).
After the mask is applied the instrument can be finished in normal fashion, spraying, varnishing or French polishing right over the bridge mask when doing the top. As mentioned, if you print out your bridge masks it is a good idea to use very light gray for the lines and text, to keep the ink from bleeding. You don’t have to worry much about the finish solvents dissolving the label adhesive. It seems to be immune to alcohol and lacquer thinner, and I’ve used oil varnish right over these too.
When you are ready to glue on the bridge it is time to remove the mask. The first step is scribing the finish around the edge of the mask so that when you remove the mask you don’t pull up the finish at the same time. This is one of those lutherie operations that require some sensitivity, but if you know in advance what it is you are trying to feel it doesn’t take long at all to learn how to do it. Basically, you want to cut through the finish all around the mask, but not cut into the soft wood of the top. I find that things work out better all around if this scribing can be done in a single pass of the knife. For polished tops, the lightest pressure will cut right through the thin and soft layer of shellac. On the other end of the spectrum is polyester finish, which is tough and generally fairly thick. Here a little more pressure on the knife is required. Gaging how hard to press the knife is easy when you go cross grain. As you begin to cut into the wood you can feel and even hear the tip of the knife blade thumping across the hard early wood of the grain. This is especially pronounced with spruce but is pretty apparent even with cedar. If you can feel this you’re cutting too deep, so back off the pressure a little.
If the finish is thin it is easy to scribe the finish freehand, running the knife blade along the inside edge between the top and the thickness of the mask. If the finish is thick I like to run the knife up against some edge when doing the scribing. For the rectangular profile classical bridge a ruler works fine as shown in photo 4. You can also use the ruler for the front edge and wing ends of a standard profile steel string bridge, but for the curved parts you’ll either have to do it free hand or use the template you use to cut out the bridge. Given my history of problems using sharp tools near finished instruments, I err on the side of caution and use the outline template I used when making the bridge to back up the blade when scribing. A knife with a thin sharp blade like a scalpel or a hobby knife works best for this operation.
After the finish is scribed it is time to remove the bridge mask. The tool I find most useful to do this is a dental probe, the one with the point bent over square. Starting in the center of the mask, the probe is poked through the paper and then run underneath, between the label and the top. I’ll generally make a slit in this way along the centerline, dividing the mask into two halves. The probe can be used to lift up the edge of the mask at the slit, and then each half can be grabbed with tweezers or the fingers and gently pulled up and away. You’ll want to take a little care that you don’t pull up any finish along the edges of the mask. This is what the scribing operation was all about. If you find that this is starting to happen, you can often set the tip of the knife in the scribed line while you are removing the mask label. This will keep the finish in place. Any chips in the edge can be glued back down by wicking thin cyanoacrylate glue under them. Of course, if they are small enough they will be covered by the bridge and don’t need to be fixed. Scrape up any bit of adhesive that remains with the blade of the knife or a small scraper.
Once the mask is completely removed I’ll generally go over the edge of the finish with a small scraper to taper it down to the wood a bit. With that last operation done the top is ready for the bridge to be glued on.
Making and using the bridge mask as described works well and is a real time saver. The only tedious part is the scribing. If I could find stick on labels made of heavier paper or card stock, it would probably make the scribing simpler. In that case the scribing knife could simply be run around the inside corner formed between the thick edge of the bridge mask and the top, no matter how thick the finish is.