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A traditional method
Filling Holes in Spruce
© Frank Ford, 2004; Photos by FF
Here's a delightful old Martin style 28 with the remains of a nasty pickguard installation. Someone screwed the thing down in three places, so now we have a valuable vintage guitar with three 1/8" holes in the face. Filing and disguising holes in light colored wood is a nearly impossible task, to say the least. Here's how I go about doing the best I can with it.
First, I cut a tiny dowel that fits the hole loosely enough that I can press it lightly in place.
With a bit of hide glue in the hole, I shove the dowel in, pressing it a little below the surface.
Now, I'd like to introduce you to a tool I made for this job. It's a VERY thin and delicate gouge, sharpened to an extremely acute cutting angle. I made it by heating the end of a piece of 1/4" drill rod to cherry red, and pounding it out like a blacksmith would. Then, I ground and polished it, heated it to bright red, quenched it in water, and tempered it in my kitchen oven for an hour at 400 degrees Fahrenheit.
After removing it from the oven, I polished it and honed it once again, to achieve this very thing edge.
On a simple handle, this gouge gives me good control.
I scoop out the area surrounding the hole I just filled with the dowel - after the glue dried, of course.
Now I have a shallow bowl shaped area that's cut beyond the area of discoloration around the perimeter of the original hole.
Here's how I did that job before I made my little gouge. Each time I was going to do the scooping operation I'd make up a "one-shot" tool by heating and bending a single edge razor blade. The heat draws off the temper of that hard edge, so while the blade is still razor sharp, the edge is so fragile it's only good for one small cut in soft wood.
But it works just fine to scoop out the spruce. Here, I'm using it to "harvest" a piece of spruce that lies along the same grain line as one of the holes in the top. Fortunately, I have the bridge off for regluing, and I have access to a perfect for a piece to fill the hole that's right in front of the bridge.
As I'm shaping the piece, I may lose my little mark, so I make sure to keep one end more pointy than the other, so I won't lose track of which way the patch should go.
After very carefully whittling the patch to fit as closely as I can, I tape it in place right where it will go, with the grain lined up as exactly as I can get it.
Then, folding back the patch, I place a drop of clear hot hide glue in the depression. No other glue works as well in this situation! Clear hide glue has the least tendency to discolor the spruce.
Folding the tape back, I clamp the patch lightly using a caul on both sides of the top to press it in place. I use a hard flat caul inside, and a soft resilient one outside. My little patch is so thin it will conform to any irregularity as it is pressed in place and I want it to fill the cavity as well as possible.
Next day, after the glue is dry, I wash off the squeeze-out with warm water and let it dry.
A regular straight single edge razor blade is just perfect for shaving the patch right down to the surrounding surface.
A bit of fine sanding - 320 grit with a semi-hard cork block to keep things level. The boundary of the patch sands away to a very delicate feather edge at which the spruce becomes almost transparent, blurring the line between the patch and the surrounding wood.
Some orange shellac for color, and the patch looks pretty good. You can see it if you look closely, but your eye isn't drawn to it because the reflectivity is just the same as the rest of that area.
Same job over by the treble edge of the fingerboard.
This area is also in direct line with the bridge, so I can salvage wood from the bridge area once again
Plug in place.
Patch shaped and ready to glue as before.
Same resilient clamping caul technique.
Shave and a haircut. (Had to say that, I suppose. . .)
And after the job is done, the hole looks more like a smudge, even in a macro shot.
Here's another tool I made. It's a sharp little gouge that cuts by pulling instead of pushing. I'm going to need it because for the third hole, I can't grab wood from under the bridge. The hole is way outboard by the pufrling on the treble side of the guitar, so I'll try to get some matching spruce from the underside of the top
I made this gouge the same way as my other gouge, but this time I used a flat piece of high carbon steel. And, once again, I could use the bent razor blade to cut wood from the inside of the top. That's what I did before I made up this tool.
By reaching inside the guitar, I can "harvest" a flake of spruce from the underside of the top near the screw hole. That's about the only way I can be certain of finding a piece of spruce with exactly the same grain pattern, runout, and reflectivity of the top in that area.
Once I get that piece out, I make certain to orient it in the same direction as it was on the underside. Then I flip the piece over, and end-for-end so that the flat surface will reflect light the same as the flat top surface. I usually have to sit and think a while as I orient the piece to make sure I'm getting it right, and then I mark it. And, just as before, I tape the piece in place before gluing.
Here you can see how clear that glue is!
Folding it back in place, I look as closely as I can to make sure the grain lines up. This particular hole had a lot of black discoloration running into the grain, so this spruce patch had to be much longer than the others.
Clamping in place, trimming and sanding the patch, a little finish touch up and the job is done.
\ The filled holes are no longer painful to look at and they will become a bit less noticeable after I do a bit of color work on the area of the top where the pickguard left a light "shadow."

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