The process of re-topping an acoustic guitar is not complicated or time consuming if done properly and with a little practice. I developed these techniques while working at Gruhn's in Nashville and use them today to replace severely damaged tops which are beyond salvage. Shown here is a guitar about to be re-topped.
First step is remove the bridge, pickguard and neck from the body of the guitar. Basic repair tools and techniques are required for this important initial step.
At this stage, the new top is glued up (joined), sanded to near final thickness and the rosette inlaid and the soundhole cut in preparation for replacing the exisiting top on the guitar.
Line up the soundholes laying the new top over the original and rough fit to the outline of the body.
Close up of soundholes.
Double stick carpet tape will hold the two tops together for the final fitting.
Masking tape protects the sides for the bearing guide of the router.
Stick the new top to the old being careful to align the centerline, turn the box over and mark the edges of the body on the new top to get a fairly accurate outline.
Close-up of the outline drawn under the new top. This excess must be band-sawn off close to the line.
The laminate trimmer cutting the edge of the new top to match the outline of the old top. It is set to cut right to the outside face of the binding
Close-up of the initial cut with the router.
Here is the final cut. The idea is to trim the top to just inside of the outside bindings of the top. The vertical depth of the cut should be the thickness of the top less 1/32 inch or so to prevent splitting the new top when routing.
Closeup of the router jig and spacer. The router does not cut deeply enough from the edge to allow for this operation without the added spacer. Check your routers for this with a tester wood block.
This picture shows the router with the space in place to give you a better idea of what is needed.
When the new top is routed to fit just inside the outside top binding and you are satisfied with the overall fit, remove it from the body and begin routing the old top. For this operation, the router is re-set to cut using a 1/4 two flute bit such that the channel the bit creates just touches the inside edge of the top bindings and cuts a swath in the top all the way around the body.
The routed channel should be the same depth as the thickness of the new top and will most likely reveal the liners and brace tails currently supporting the existing top. (Shown)
At this point or somewhere before this point, you brace off the new top to match the bracing on the old top (or modify it as needed) and prepare it for the replacement process. Cut the brace tails to the very edge of the fitted to rather than having any overhang as they must fit the existing liners under the old top.
When the new top is all braced and ready to go on, remove the existing top by gently tapping it from the underside with a small jeweler's hammer or similar device. A hot knife will remove the top from the end block areas. Be careful when releasing the brace tails as they can split out the sides if not done gently.
Drop the new top into the space where the old top was, fitting the brace tails as needed and making any small adjustments to assure a tight fit. The sides may splay out a little while the old top is off and before the new one is in place, but that is not a problem at all. The sides will all pop into place and everything will line up when the top is clamped into place with a little tape.
A close-up of how the new top fits into the space of the old. It should be a very tight fit between the top and the inside edge of the old bindings.
Glue the new top in place and use tape or elastic strapping to pull the sides tight to the top.
The next step is to route the channel for the purflings of the top and pop them into place with glue. By being careful in the setup of the router for this operation, it is quite easy to have a perfect channel for the purflings and an invisible joint. The effect is to make it look like the new top was always the top for this guitar and nothing had ever been changed about it. The last steps include the refinish of the top and re-assembly of neck, bridge, and pickguard components, string up and final buff out.
The finished guitar back in service. Total down time is about five days.