MIMF

Sorry to raise this again, but... I used to frequent here years ago and am getting the itch to build again. Seems to me that there were several builder who advocated that it was OK to glue the bridge on top of the completely cured finish of a steel string guitar. I am wondering if this would work on a classical with french polish or if one would resort to a modern varnish.

Does anyone glue on top of the varnish or has it failed the test over time???

This is not meant to reopen old wounds, just to have a healthy discussion.

Thanks!
Rob

Welcome back, Rob.

I am not familiar with anyone that glues bridges to the finish except for the late Rick Turner at Renaissance Guitars. I think he used medium CA glue. However, I would not recommend the approach.

It is relatively easy to scrape the finish off the bridge footprint. Also, the technique of routing a small rabbet around the bridge perimeter and letting the finish slip under the bridge makes for a cleaner install.

Also, no need to apologize for raising a topic that has been discussed before. That is what the forum is here for.

AH yes, Rick Turner! would be interesting to know how many of the guitars he made were returned for repair (If any!)
Thanks for the reply.

Ovation glues (or used to) the bridges on top of the finish. In their case, the finish was a full millimeter (.040") of epoxy. It's very hard, and difficult to remove gracefully with hand tools (DAMHIKT). Their bridges still do come up.

FP is easy to remove, especially if you've masked off the central area of the bridge beforehand.

Some folks do glue bridges on top of UV cure polyester. The few that I know use CA, and seem to have pretty good luck with it, so far.

If you are indeed planning to french polish, it is very easy to mask the bridge area before hand.

I put down masking tape over the bridge area, locate the bridge and score around it. You need to be careful and use a sharp blade (I use a scalpel) so that you are only cutting the tape and not wood fibers. Then I peel off the tape leaving the bridge area covered. I just french polish right over the tape. The finish is thin enough that once you are done (I leave it sit for a week or so to harden up after I'm done [usually. . .]) peeling the tape up carefully does not damage the finish transition. I then scrape a very narrow and shallow rebate around the bridge perimeter. Even though the full foot print of the bridge has been masked and is finish free, it is nice to not have to worry about the corners of the bridge footprint interfacing with the edge of the finish. Scrape/clean/prep the top and the bottom of the bridge and glue down.

I do it similarly, except I mask a slightly smaller area, which reduces embarrassment if you misjudge the location a bit when taping. Scribe around the bridge and clean up just before you're ready to glue.

I'm not a big fan of the rebate around the edge. The maximum stress on the glue line is along the back edge, where the shearing force of the tension and the torque due to the fact that the strings are above the plane of the top add up. The shear stress is low in the center of the bridge, and rises fast right at the edges. The greater the depth of the bridge along the line of pull the lower the max stress at the back edge, since more of the load is taken up in the center. That's why the 'belly' bridge works: the overall area of the footprint is not as large as that on many Classical bridges, but the width reduces the stress disproportionately. Even a small reduction in width is unhelpful, so I try to maximize it by gluing right out to the edge of the bridge.

My rabbet is only about 0.010" to 0.015" wide so there is a minimal loss of real estate.