Now that I'm confident that I've set the correct neck alignment, it's time to refit the neck. I'll prepare a couple of mahogany shims and sand them to a taper by holding them on my coarse sanding plate and block sanding from above with finer sandpaper:
I'll slip the shims into the dovetail pocket and jam the neck in on top of them.
When I take the shims back out, there'll be shiny pressure marks where they fit against the sides of the joint.
I can also use carbon paper, chalk, or dentist's articulating paper to highlight these areas, but here the shiny spots are very clear. I'll scrape and block sand the shiny areas and try the joint again. By repeating the process I'll eventually get the shims to fit very tightly with the neck sitting correctly in the joint. At this point I'll recheck with my straightedges and if everything's all right, then I'll glue and clamp the neck with cauls to protect the fingerboard and body.
I used hide glue to reassemble this vintage Martin because that's the glue they used when it was built, and that's the glue a future repair tech would expect to encounter.
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