Martin Cracked Bridge Repair - Cracks Between Peg Holes

Replace or Repair

Bridge repairs on Martins are not unusual and typically involve removing the bridge and replacing the piece.  In this case there is a crack in an ebony bridge between the peg holes.  

Many luthiers would replace this piece since it is likely at some time a replacement will be needed. There are a few things to check to see if replacement is needed.  Here is what Dan Erlewine has to say about this repair- seems to say you can fix or replace:

The bridge cracks occurred from: 

1. Pressing the pins in to hard - see the Martin 'care instructions which reference this as a cause
2. Using the wrong bridge pin size
3. Top warping from dryness which will make the top convex - the center sinks more than 1mm.  This will cause the bridge to separate from the top usually along the back of the bridge (opposite the sound hole).  Another symptom of convex warping is string buzz since the bridge is now lower to the neck.
4. Top belly forming in the area of the bridge that is extreme enough to cause the bridge to stress and separate. 

Other considerations:

• If the bridge is lifting then it should be removed and reglued or replaced.  
• If the bridge has lifted then the bridgeplate support under the bridge inside the guitar may have separated from the top also.  The condition of the bridgeplate should also be checked.
• If the top is warped also check the back
• Do not correct the belly deflection.  Most spruce tops and other woods form a bridge bulge which contributes to the aged tone. 

OK so what needs to be checked:

Has the bridge lifted?  Check this by using wax paper or the flap of a paper string envelope and see if you can slide it under the bridge along the back, sides, corners and front.  If you get the paper to go in more than a millimeter or two then consider removing the bridge and regluing. In this case the bridge is firm with only small area under the corners that has separated.

Condition of the bridgeplate- In this case firm and in good condition with the bottom holes the proper size.

Top warping- The top is warped as shown below and it was corrected with several days at 50% humidity.

Here the low points are about 2mm and less at the center bulge

After several days wrapped in plastic with internal humidifiers
the maximum drop is 0.8mm and the center is slightly higher than
the sides

Fixing the Cracks

The steps are the same as repairing other small wood cracks:

1. Fill with sawdust
2. Fill with thin cyanoacetate glue
3. Sand
4. Oil finish to match 

Sawdust from a piece of ebony.  CA is thin and applied
with a thin wick top from Stewmac

Tape off crack area and place paper towels inside
guitar under peg holes to catch glue and saw dust

Fill in cracks with sawdust

Multiple applications of CA and sawdust building a
small bump over the cracks. Apply several layers 
waiting 10-15 minutes in between

After sanding with 600 and 800 wet dry paper and a few drops of mineral oil

Martin Acoustic D-41 Binding Repair

Loose Waist Binding

This is a 1996 Martin D-41 (purchased new from Mandolin Bros Staten Island) and a few years ago the binding came loose and I took it to someone for repair.  A few weeks ago it let go at the waist and there is shrinkage of the binding so it can not simply be glued back in place.  I decided to fix this myself since the last 'professional' did not do a good job and it likely came loose and shrunk due to the lack of glue.

There are a few options where you can cut it, glue it then patch but ideally this should be done at the base of the neck or the tail.  

Here is a link and best reference I could find for the repair.  It is very complete and in this case described in the link the binding broke when trying to free it from under the neck.  I took a different approach and decided to cut it at that point so I had a clean edge to work with for the patch.

http://www.frets.com/FretsPages/Luthier/Technique/Structural/ReglueBinding/regluebinding.html


Loosen the Binding

The binding was fairly loose between the waist and neck.  It lifted easily using an exacto knife until I got close to the heel of the neck.

I scored the binding along the neck and seams close to the neck so it would not pull away any of the wood or other layered bindings as it was cut and lifted from the binding channel.

Prepping for Glue

The channel and back of the binding was fairly clean and I did not want to risk cracking it so I did not scrape the wood binding channel or use glue solvent on the back if the binding.  There was little or no old glue here so I skipped it.

Glue and Tape

I used Titebond III with a Stewmac glue applicator to get the glue into the channel.  First I placed tape at a portion of the binding that was firmly in place so it would not loosen when the binding was bent to place glue in the gap.  Glue was placed in the entire length of the channel.  The following steps were used to secure the binding in place:

• Press the binding in place for a few inches.
• Clean the excess glue with wet paper towels then dry paper towels.
• Place painters tape on the side and firmly pull the tape over the corner and on the top to secure. Use thin pieces of tape.

I worked my way from the waist to the neck leaving a small gap at the neck that needs to be filled.  I left the tape in place for 24 hours which is the full set time for the Titebond.

Gap Filling

I checked with Martin service and they confirmed the 1996 binding is Boltaron which is an ABS plastic. I ordered material from Amazon 1.5mm x 5mm and will get back to this when it arrives.

The gap is small- maybe 1/8 inch.

The binding material dissolves in acetone so the end of a piece shaved thin with a knife then softened in acetone. The soft end is placed in the gap then cut with a single edge razor blade.  The material hardens for 24-48 hours before final trimming.

Shaved binding

Softening with acetone

Pushing into gap

Filled

Smoothing

The rough edges are trimmed with a single edge razor blade using scotch tape to protect the lacquer on the body. After the smoothing with the razor it is rubbed with 800 wet paper.