One of the more challenging clamping tasks is that of securing the acoustic guitar bridge to the top plate of the guitar. My preference is to reserve this operation until one of the last steps in completing the guitar. I like to have the instrument completely finished and rubbed out, the nut installed and all the other detail work complete before securing the bridge. Also, the bridge should be complete, including pin holes, tapers, rounding etc. Finishing the wood surface of the bridge with shellac, lacquer, oil or water-based finish is optional.
Begin by carefully placing the bridge on the top and measuring from the backside of the nut to the center of the saddle slot between the 3rd and 4th strings. If you like to make things a bit easier the bridge can be lightly tacked to the top with a couple pieces of double sided tape. Don’t press down on the bridge to secure the tape until the bridge has been accurately placed both horizontally and vertically.
Always double check the dimension to make sure the bridge is exactly placed to the scale length.
When the bridge has been secured, use an appropriately sized drill bit to drill down through both the high and low “E” strings. You should drill down through the top plate and the bridge place. To prevent splintering on the underside of the bridge plate a block can be temporally placed inside the guitar to minimize this.
Next the outer edges of the bridge need to be accurately transcribed on the guitar top. I find this easiest with painters masking tape. It will stick to the top but removal is easy and it won’t damage the finish. Remove the bridge when the perimeter of the bridge has been carefully masked off. Note: If possible, try to place the tape line about 1/32″ or 1mm beneath the bridge edge. This will allow the finish to extend very slightly beneath the bridge outer perimeter.
If the top has been entirely finished, now is the time to scrape off the finish beneath the bridge or the bridge gluing surface. Be very careful in doing this. One trick you can use is to use a sharp knife to cut a clean edge of for the bridge on the straight lines. The curved lines will have to be cut by hand. Use a sharp, small steel scraper for this and scrap the finish off down to the bare wood. Sanding should not be needed if the scraper is sharp. Optionally use 220 grit sandpaper with a small wood block and switch to folded sandpaper for the final sanding.
The bridge is now ready to be clamped to the guitar top. By using the GLS Guitar Bridge Clamp, no supplemental clamping is necessary. I prefer to use 2 part slow set epoxy to glue my bridges. Be sure to clean the bottom of Rosewood bridges with Acetone until color no longer comes off on the rag. Mix up a batch of epoxy and spread a very thin layer on the bottom of the bridge using a straightedge of a small stick or wood veneer.
Place the bridge in its correct location and place the Bridge Clamp on the bridge top. Align the holes on the clamp with the bridge E string pin holes and drop the screws through the holes. I like to use a bit of paraffin wax on the underside of the bridge plate to prevent any stray glue from gluing the bridge bolts to the bridge plate. You can also lubricate the bolts and washer to achieve the same results. Slide a small lock washer over each bold end inside the guitar and finger tighten the bolts. Reposition the bridge is necessary. Tighten the bolts from the top with a phillips screwdriver. Use adequate force to see some glue squeeze out around the bridge.
Next position the end aluminum, cork-bottom clamping cauls on the bridge end flairs or wings. Position the caul beneath the screw knob and tighten each side of the clamp. Note that extreme clamping pressure is not needed on the ends and this may deform the top slightly. You should see glue squeezing out around the entire perimeter of the bridge. Clean the glue carefully from all areas of the squeeze out. Carefully remove the tape when the glue around the bridge is completely cleaned up. You will want to finish this before the glue hardens.
Set the guitar aside to dry overnight. Remove the GLS Bridge Clamp and drill the remaining 4 bridge pin holes. You may need to re-drill the out bridge pin holes due to glue squeeze out as well.