Arlington Fret Works

 
This 1973  Washburn 12 string had a few bridge and saddle issues.  
 

The saddle had fallen forward from 36 years of string tension.  All the parts were loose fitting and started tilting, then the saddle cracked and collapsed.  The bridge was also lifting quite a bit. I could slide paper under it right up to the pins, and also around the edges.  That's a lot of glue failure, so this bridge would have to be removed and re-glued. I suspected the guitar had been sprayed with finish before the bridge was glued on, which makes for a weak joint.

First thing to do is to apply heat to weaken the glue bond. A heat lamp works, but you have to keep moving it, and don't walk away!  Cardboard covered with aluminum foil shields the rest of the guitar.

After about 8 minutes under the lamp I start to work a few sharpened putty knives under the bridge. Junk mail works great for a bit of leverage and to protect the top. I keep working from different angles, prying gently and cutting the softened glue with the knives.  

As I suspected the top was finished under the bridge.  I will have to scrape all the glue and finish off both top and bridge.

A friend recommended this shape of pocket knife for scraping, and it works great. The big blade has a nice curve as well as a flat area and a good place to push sideways with your thumb.  I keep scraping until the shavings turn brown. The white shavings are old glue.  I also check the  bridge with a small straightedge and scrape until the entire bridge is flat from front to back. This bridge is curved side to side to match the curve of the top.  I didn't want to change that, so I hand scraped instead of using the flat belt sander.

To scrape the top I use a razor blade, a chisel held straight up, and my handy pocket knife.  I had lightly scribe a line in the finish tracing the footprint of the bridge, and I carefully scrape just short of that line.  It took almost an hour of careful scraping to get down to a clean surface for a good wood to wood glue bond.

I made these special cauls for gluing bridges. The top caul is stiff hardwood with two thumbscrews that act as secondary clamps. With this caul I only need to put one clamp through the sound hole.  The caul for inside the guitar has notches for the bracing and is slightly convex to match the curve of the bridge.  

 
The trial fit. The pins align the bridge, and I use masking tape to mark the edges.  This way I can still see the line when the glue starts to squeeze out.

I'll leave it clamped for a day then wait two more days before putting the strings on.

The wood swells a tiny bit from the wet glue so I use a reamer to clean up the bridge pin holes. Now it's ready for a new bone saddle and strings.

This guitar has an adjustable metal seat for the saddle. Both were loose and the saddle tipped forward and broke. First I decided to try to add a shim to the metal part.

I glued on a thin piece of veneer with crazy glue, then sanded it down until the part fit snugly in its slot. I ended up with .0012 inch of shim. I'm not sure this was worth the effort, but it stabilized it a bit. 

The new bone saddle fits tight, and stays upright nicely.

Here's another not so great glue job on an Ibanez classical guitar from the 70s. The top had been finished first, then just a few squirts of glue applied: