Glueing exotic woods

Since repairability is important to me I can't use any epoxies or similar glues.

I believe this is an assumption and one you should challenge.

In simple testing examples certainly, reversing "wood glue" (PVA, both yellow and white) and various protein glues can be much easier when compared to an epoxy (especially a high-end epoxy).

However, in context within joints in a piece, there's scant evidence that the joints will always be easier to reverse if not done in epoxy¹. Or to put it another way, since this is often stated, that the epoxy joints will be literally impossible to separate; this is simply not true.

Epoxies in general have low or lowish glass-transition temperatures – they soften with heat. And getting heat into a joint is not that difficult to envisage.

One of the oft-mentioned advantages of the other adhesives you mention is a liquid of some kind will dissolve or at least soften the dried glue, which is entirely true. But this approach still faces a considerable obstacle: how do you get that liquid into the joint in the first place? And what havoc might all that liquid do to the wood, finishes etc..... the respective solvents for PVA and protein glues being acetone and water or vinegar (which is almost all water).

So the question would be what the hardest wood is that you have ever succesfully glued (stressed joint - not failing even after long time) using normal wood glue or maybe hide glue.

I have one example type of highly stressed joint in oily/resinous exotics that is worth you investigating further, and that is repair to the extremely common break in the middle of the rear handle of hand planes².

For a lot of people (including myself) the adhesive of choice here is epoxy, however, many just use the glue they use for everything else and there are numerous reports of zero failures over time as you'll find.

Don't focus exclusively on the adhesive choice
In general however, the main 'secret' to successful bonding of problem woods comes down to prep as I'm sure you'll have already read. The 6-P rule most certainly applies to glueing!

Many people solvent-clean³ exotics immediately prior to applying adhesive and this certainly works to degrease the surface (you can see it with the naked eye, as well as confirm it has worked by testing). This by its very nature guarantees you get a better join in the long term over the same joint glued without any prior intervention, irrespective of the adhesive used.

Two other basics to bear in mind if using PVAs that I wanted to mention:

• PVA is not a gap-filler like epoxy. Even on a micro level PVA doesn't like gaps. So very smoothly prepped surfaces are best if using white or yellow glue.

• PVA requires hard clamping pressure to achieve the maximum hold it is capable of. This is non-negotiable. Almost everyone (genuine pros included) under-clamps joints made with PVAs but get away with it, simply because most joints aren't under tons of stress. You will want to err on the side of caution and clamp hard; go to town on the number of clamps used and don't be afraid to crank 'em down!

¹ As should be obvious, especially if comparing to PVA rather than a protein glue.

² In a user plane this repaired joint is put under strain repeatedly and sometimes over an extended period, and they are required to withstand this for years if not decades afterward.

³ Generally using an alcohol or acetone. I would generally recommend using acetone because it evaporates a lot faster, and you want to proceed as fast as possible doing this.

"Standard" instrument-making procedure uses hide glue. That seems to hold violins/cellos and the like together for decades-to-centuries. Comes apart with steam and a bit of leverage. Give the timber a bit of "tooth" by sanding it rather than a gloss finish straight from the plane and the glue should stick pretty tight. The neck joint is probably the point with most stress applied to it, and if you're really worried, a screw or bolt in addition to glue might take the stress off the joint. A bit of metal hardware on the back of the body isn't going to ruin the aesthetic any more than what is on the front: Bridges are usually screwed on, tuning keys are shiny metal, and strings are steel, and so on.

As to exotic woods, test gluing with a range of glues on a spare piece of timber and load testing it might be worth a bit of experimentation before committing to a full build. (Maybe twisting it rather than hanging a weight off the glued pieces might be more appropriate for a guitar.) Are your particular timbers exotic because they are oily and difficult to glue and woodworkers tend to avoid them, or is it a sweeping statement that anything you've never worked with before is "exotic" and that includes (but is not limited to) oily timbers?

A third point is how much repair do you plan to have to make on this guitar? What are you going to do to it that requires repair before you've even made it?