Martin N-20 Folk Guitar repair

[Barry Daniels]

Up next on the workbench is a Martin nylon string guitar from 1969 that has some of the nicest Brazilian Rosewood I have seen. It has not been played much and suffers from common issues including back cracks, shrunken binding and a broken transverse brace on the top.

[Barry Daniels]

I repaired the broken transverse brace and related top crack, added a cleat and did an initial finish touch up that will need leveling and buffing.

[Barry Daniels]

The back binding had pulled away from the waist on both sides. I was able to loosen them up to the neck and reglue them. This will allow me to only have a small piece that I have to add at the top behind the neck heel cap.

[Barry Daniels]

The real reason I am posting this repair is a new method for repairing back cracks that I am trying for the first time. T.J. Thompson sells crack repair tools that are very innovative. Stew Mac sells his kit for top cracks, but if you want his back crack kit you will have to look at his own website:

https://proluthiertools.com/product/bac ... epair-kit/

The stated advantages to this tool is that clamps are not required due to the large rare earth magnets. Also, the kit includes a continuous cleat that should provide better support to the crack. The rapid placement of the two cauls (the inside caul has the cleat stuck to it) allows you to glue the crack and the cleat in one operation using hide glue, if you wish. I am excited about testing it out.

I ran an initial test on a scrap to see if there would be any problems removing the cauls after the glue sets up. When the cauls are placed on either side of the plate they pop together immediately giving a satisfying pop. I tried to use just an adequate amount of HHG but the pressure on the joint caused quite a bit of squeeze out. We will see tomorrow if they come apart easily.

[Clay Schaeffer]

Hey Barry,
I found the T.J.Thompson continuous cleats inspiring. Since they span the entire length of the cleat I'm thinking they don't need to be too thick. I decided to experiment with using veneers. I cut the veneer strips with scissors. Since the grain runs perpendicular to the crack, cutting with scissors tends to make the veneer break into smaller segments. I placed a piece of masking tape over the strip before I cut it off the length of veneer so the segments stay aligned. I leave the tape on until the strip is glued in place and set up. This also keeps the caul from being glued to the veneer strip and the tape peels off easily from the veneer.
The magnets look like they would be a nice addition, so I will have to see what I can find.

[Clay Schaeffer]

P.S. Some people like to convert the G series classicals to steel strings - the attraction is the nice rosewood that was used and the G series body is shaped more like the steel string guitar. The N series has the typical classical shape - it would make a Gibsonesque LG type steel string. Martin classicals aren't highly regarded in the classical guitar world for some reason.

[Barry Daniels]

The cleats are 3/8" wide and .056" thick, with a rounded back profile. They are small compared to the normal diamond shape cleats but should be sufficient to reinforce the crack. The magnets put a lot of force on the joint giving a very solid joint. My test piece turned out fine. The HHG did not stick to the acrylic cleat. You can see where the cleat first hit the back piece and then it slid into the proper position, leaving a smear of glue.

[Barry Daniels]

A bit of cleanup with a damp rag and it looks good. There are no visible gaps in the joint.

[Barry Daniels]

I am happy with the results so will proceed with the actual repair. The first step is to map out the brace locations with a couple of magnets.

[Barry Daniels]

And then locate the outside caul. The inside caul is a little bit long and needs to be trimmed to fit between the braces, or in this case, the lowest brace and the kerfing.

[Clay Schaeffer]

neodymium magnets.jpg (103.23 KiB) Viewed 18122 times

I didn't think about mapping out the brace locations. Thank you for showing that. You mentioned you need to trim the caul - are the cauls integral with the magnets, or can you buy additional cauls and attach the magnets to them?
I was looking into buying some magnets to do something similar with the veneer cleats. It looks like a lot of places carry them and they are not too expensive. Then I remembered I had a bunch of tiny ones I salvaged from a project I was on. I'm thinking I might mount them in some UHMWPE or Corian plastic - make squares a little bigger than a chiclet and hold them together with masking tape. That would allow me to vary their length to suit the need.
I wonder if they would be strong enough to glue a bridge? Maybe make a flexible caul that would span the bridge with magnets on both sides...

[Barry Daniels]

The magnets are stuck to the cauls with double stick tape. As far as I know, you can only buy the cauls and magnets as a kit and it includes three sets of different lengths. The magnets are arranged so that they essentially act like one big magnet.

[Barry Daniels]

More progress. I can't reach all the way to the bottom of the guitar so I made a handle and attached it to the back of the magnets with the ultra strong 3M double sided tape.

Then place double stick tape to the groove in the inner caul where the cleat fits.

[Barry Daniels]

Press the cleat into the goove.

[Barry Daniels]

Trim the cleat flush to the ends of the caul.

[Barry Daniels]

Warm things up, smear some HHG on the crack and work it in, clean off the glue, tape the outside caul down, apply glue onto the cleat and place it in the guitar. This all takes about as long as it takes to read this list.

This is so much quicker and easier than traditional back repair methods. I expect it to be more robust too. An additional benefit is the edges of the crack are forced to be level by the magnets. So far, I am impressed.

[Clay Schaeffer]

When I first saw the system it looked like it would work pretty well. You have shown several of the things to be taken into consideration when using it.
It builds on the the idea many of us have used for crack repair. Previously we used magnets only to align and level cracks. Now they are being used as a clamping system and in addition putting the cleat in place. Pretty slick!

[Barry Daniels]

One caution: the magnets are really strong. So the outside caul can scratch your finish when moving it due to the extreme down force. I think a patch of cloth under it would be a good precaution.

Also, the cleats are really small and would be difficult to machine yourself. Buying them from ProLuthierTools is probably the best option. He makes them in rosewood, mahogany and spruce. The latter making this tool useful for top cracks.

Another question I had was how does this work on arched plates. I discovered that the cauls are flexible enough to easily conform to a back arch. But they might not be able to work on side cracks except for the relatively flat area below the waist. Or you could split the caul and cleats up into short pieces and do a side crack in sections.

[Clay Schaeffer]

If you buy some neodymium magnet discs or squares from Amazon or the local big box store you could mount them to a cloth strip and leave a small space so they could flex between the magnets and conform to the side of the guitar. As I mentioned I was inspired by Thompson's system to use veneer strips for cleats, held together by masking tape until they are put in place. A light spritz of spray glue (maybe one of the cheap low strength craft glues) could be used to hold the veneer/tape strip to the magnet strip until the glue has set. Veneer may not look quite as nice as the Thompson cleats, but I think could work well, and with a little bit of sanding (didn't someone suggest using an aquarium glass cleaning magnet set to sand inside the guitar) could look O.K.

[Barry Daniels]

Thompson's cleats are .056" thick so I think you might want to double up on your veneer strips to get an equivalent strength. I will have to do some tests with Thompson's cauls to see if I can get them to flex against guitar sides. Making an extra flexible caul is an interesting idea, Clay, because side cracks are very common and clamping them is next to impossible without magnets. Also, alignment of side cracks is often problematic.