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Shimming Guitar Neck Dovetail and Finish Chip Repair: 1931 Martin 0-18T Restoration and Repair, Pt. 4

We now have the correct neck angle (aka correct set or pitch). Since material has been removed from the edges of the heel where it meets the body, it doesn't quite follow the curve of the sides of the guitar.

To get a nice fit, we hold the neck in place, and pull some sandpaper through the joint, facing it upward so it removes material from the heel.

If your fit is close, it won't take many more than 6 or 8 pulls on either side.  It's important that you watch to make sure the sides are level. I usually do a couple of pulls on one side, then the other. If you're not careful, you'll introduce a 'tilt' of the neck from one side to the other, and potentially mess up your nice neck set.

I used to use regular 100 or so grit paper for this. But normal paper is thin, and it will tear and break. So I sprung for this paper from Stew-Mac. You get a lot for the money, and it's heavy so it won't tear.

The fit in the dovetail is pretty good, but I want to get it a bit tighter, so I made some thin shims from cedar and glued them to the dovetail.

The shims are thin - maybe .050 of an inch or less.

I had previously made angled cauls for this job.

After the glue is dried on the shims, I put some carbon paper over the shims and fit the dovetail to the guitar. The paper will leave marks on the shims where the shims contact the dovetail.

You can see the marks here.

Then we use a scraper to remove material from the dark marks left by the carbon paper.

Repeat this process until you have a tight fit on the dovetail. The joint should be tight enough that the guitar could be strung without the joint being glued. The reason for that is transfer of vibration, which means good tone.

It will take me 6 to 8 passes like this until I have a good joint where the dovetail seats with a little pressure, stays in place on its own and doesn't wiggle from side to side.

Two things here.

One, you can see the final scraped-down shim. It's really thin - so thin that parts of the shim are gone.

Second, I stupidly chipped a hunk off the neck heel while scraping.

I'm not happy, but I can repair that chip.

File some mahogany sawdust with a rasp.

I used to use sandpaper on the wood for these fills, but you tend to wind up with sandpaper dust in the wood dust.

Pack some mahogany dust into the chip.

Then hit it with thin CA (super glue).

You can see the glue will help darken the color of the fill.

The fill material will tend to sink down as the CA is applied, so it may take a couple applications to fill the chip. This one took three passes. I like to leave the fill a bit 'proud' of the surface so it can be levelled.

Then I'll use some yellow/amber binding toner nitro lacquer I made up. The original lacquer used on the guitar was clear, but it's now yellowed over time, so we can emulate that color to match the original lacquer color.

Scrape and sand the repair so it's flush with the surrounding wood.

I used artist's tape to cover the area around the fill since I don't want to sand that area.

Then put several coats of the toner on the repaired area. It won't be totally invisible, but it won't be obvious unless you're 10 inches from it.

The lacquer can be sanded and polished as you usually would after a week or two.

All posts in the 1931 Martin 0-18T Restoration Project:
  1. 1931 Martin 0-18T Guitar Restoration and Repair, Pt. 1

  2. Neck Removal on 1931 Martin 0-18T Guitar: Restoration and Repair, Pt. 2

  3. Trimming the Neck Heel for Reset: 1931 Martin 0-18T Restoration and Repair, Pt. 3

  4. Shimming Guitar Neck Dovetail and Finish Chip Repair: 1931 Martin 0-18T Restoration and Repair, Pt. 4 - This page.

  5. Caul for Heat-straightening Guitar Neck: 1931 Martin 0-18T Restoration and Repair, Pt. 5

  6. Reparing Acoustic Body Cracks: 1931 Martin 0-18T Restoration and Repair, Pt. 6

  7. Making a Tortoloid Pickguard: 1931 Martin 0-18T Restoration and Repair, Pt. 7

  8. Heat Straightening Bowed Guitar Neck: 1931 Martin 0-18T Restoration and Repair, Pt. 8

  9. Fret Marker Installation and Filling Fingerboard Chips: 1931 Martin 0-18T Restoration and Repair, Pt. 9

  10. Compression Fretting to Correct Upbow: 1931 Martin 0-18T Restoration and Repair, Pt. 10

  11. Completed 1931 Martin 0-18T: Restoration and Repair, Pt. 11


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