I wonder what your experience is with sunken tops on (vintage) GJ guitars, is this a common thing in your experience, and if and how this should best be repaired.
I see a lot of sunken tops on vintage GJ guitars. They sink or can or will sink at two places. At the bridge and at the upper part of the top near the sound-hole. My local luthier (he has previously worked at Dupont) says a lot of vintage GJ guitars have been build quite lousy, the lower brace is often not placed in the right spot, so that the top at the bridge sinks in time. Often there is not must support around the sound-hole. Often the braces that are placed under the top have not been sanded correctly (to give them a (slight) arch), they are too flat which will make the soundboard sink in time. Another thing is that a lot of the vintage GJ guitars have had wrong heavy gauge strings on them in the past, these guitars are not build for that and that makes the top sink too. I think you should think of GJ guitars as something in between a classical nylon string guitar and a steel stringed folk guitar when you think about the pressure a GJ guitar top can handle. Which brings me to the question how you should best treat them when you repair a GJ guitar. For exemple if you want to do a neck-reset. I see more often problems with the top than with the neck, when action is too high, most people would say neck reset, but in my opinion the problem is not the neck which causes the high action, but the top.
The ones that have not sunken have bigger braces and the lower brace is placed nearly exactly under the bridge. So that makes me think of a probable cause of the sunken tops: the braces. So is this where the root of this problem lies?
Changing the braces would affect the sound of the guitar from what i understand, so this is not something to consider lightly and comes with a certain cost and if you like the sound of your guitar you might not want this to change.
Another option could be taking off the braces and resand them so that they become a bit arched. Does anyone has any experience with this?
I know about the classical neck reset, but I am not sure this really fixes the top.