The back was almost totally off the 1890s "University of Michigan" parlor guitar. All of the back braces were loose (only at the ends, not in the middle,) so I took the back off to fix the braces. Under the braces, the Brazilian rosewood is scored with multiple fine lines. These line run across the grain of the rosewood, and in the same direction as the braces.
My dad, an accomplished woodworker referred to this as giving the glue joint "tooth." The original makers of the guitar must have thought that there was some benefit to this practice, or they wouldn't have taken the time and trouble to do it.
On the other hand, all the back braces were loose. Or perhaps they would have been entirely off without the tooth.
Anybody still doing this? Any ideas?
Paul, what a great tool/tip! Thanks for posting this..
Thanks to everyone for all the input.
I have always added tooth to the backs of ebony bridges using a sharp rasp in the same direction as the grain. Never even considered doing it to a spruce top.
As a dyed in the wool traditionalist, I'll probably continue the same with the ebony bridges, hoping that Paul is right and that "those old guys were on to something."
I've heard it said that "toothing" the surfaces helps in keeping the parts in alignment as the clamping pressure is applied. That may be why we see those marks on old glue joints where alignment is critical and no fixturing was used. The most stressed joints of a guitar, for example, are the center joints of the top and back, and you never hear talk about roughing them. Additionally, there aren't toothing marks on other joints, such as neck and end blocks.
Frank, FWIW, the University of Michigan guitar has the back scored with tooth marks under both the neck and tail blocks, and even under the kerfing. May be just another instance of "If a little is good, more is better."