Official Luthiers Forum!

The patch, any patch, is there to keep the 'open' side of the X joint from spreading. The down load of bridge torque pushes it open, and that concentrates bending stress at the bottom of the lap; a large stress riser. The brace can split at that point, as I've seen a number of times. Any run out in the brace wood can cause it to split down toward the top.

A friend of mine who had lots of repair experience in the violin world pointed out the he had often seen a line of studs along an old crack, with a new crack along the ends of the studs. The studs were too stiff to move with the wood, and produced a stress riser. He'd also seen lots of studs that peeled up, again, because they were too thick and not tapered enough to move with the wood. He had never seen a broken stud.

The patch here is under a tension load, but not a very high one, and doesn't need to be very thick. Try pulling a half millimeter thick spruce patch 6mm wide apart with your hands. What it does need is a long enough glue line along the line of pull to stay glued down. I suspect that a piece of paper across the top of the joint would do, but haven't wanted to do the necessary tests to prove it. Cloth works well under tension, of course.

The problem with glued on cloth and paper, which I've seen on plenty of old instruments (although not yet on an X crossing patch) is that air getting at the glue breaks it down over time. You see lots of old 'tater bug' mandolins with the paper lining in the bowl flapping loose because the glue deteriorated. I use cloth side tapes, but I give them a couple of coats of shellac to seal them from moisture. in the hope that it will help. We'll find out in 75 years or so whether it worked.

I suspect that Martin used linen gun cleaning patches because they could get them easily. It's easy to work in a bit of glue and form the patch over the joint while it's still worm. Done.