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One thing I took away from years of repair experience is that the bridge needs to be glued to wood, it needs to be glued over it's entire footprint, and you need to be really careful about cutting into the top wood, particularly along the back edge of the bridge. Most bridge reglue jobs are due to messing up one or more of these three things.
Technical background:
There are two forces on the glue line between the bridge and the top: a shearing load, as the bridge tries to slide along the top toward the neck, and a torque caused by the fact that the strings come off the saddle above the plane of the top. The torque load is trying to peel the bridge off the top from the back edge, and most glues are not really very resistent to peeling.
The shear load is a little harder to characterize. In a book entitled "The New Science of Strong Materials; or Why you Don't Fall Through the Floor", the author gave a diagram of the force. Basically it's very high at the front and back edges of the bridge, and falls off rapidly to a low level for most of the width. The area under the curve is the total force trying to slide the bridge along the top. The height of the level part of the curve is set by the nature of the glue line and the woods involved. Assuming a good glue line, the greater the distance between the front and back edges of the bridge, the more of the total force will be taken up in that low stress central portion of the joint, and the lower the peaks at the front and back of the bridge will be. When the peak load along the edges of the bridge exceeds the limit load of the glue line, the bridge comes up. That's one reason why a belly bridge can take more tension on a smaller footprint that the narrower, but longer, bridge on a classical guitar.
When you sum the two forces up, you can see that the critical part of the glue joint is along the back edge of the bridge, where the peeling and shearing forces both are high, and are working together to take the thing off the guitar top.
Leaving a ledge of any sort around the bridge is the equivalent of making the bridge that much narrower, and raises the maximum force along the back edge. This is especially dangerous if you're just leaving an area of finish under the edge, with the finish scraped away from the rest of the top, and the bottom of the bridge is flat. Not only have you reduced the area and the distance along the line of pull, but you've made it impossible to clamp the bridge down to the wood, and thus made it more likely that you'll have a too-thick layer of glue there. This is an all too common thing on lower priced manufactured guitars, where they mask off the bridge area undersize, and just peel up the tape before gluing the bridge down. I did, however, see this on one Martin some years ago; a real shock.
We once had a Lowden for sale in the shop that simply shed it's bridge one afternoon. The glue line had not failed: there was 100% wood coverage on the bottom of the bridge. Whoever had scribed around it to scrape off the finish from the top had probably gotten a new blade in their utility knife, and had cut into the fibers of the top all the way around. Cedar had a low peeling resistance in itself, and once the crack got started the stress along the back edge rose, making it go faster. Cut the finish, not the top wood. I like a sharp knife and light cut.
Ovation epoxies the bridges on over the epoxy finish (about 1/16" thick!) on the tops. The bridges still come up, and they usually take some top with them. Most glues don't stick to the underlying finish as well as that, so I would not expect a bridge glued on over finish to last very long.
I don't understand how you can rout off the finish without cutting into the wood of the top. This is bound to leave a stress riser: a sudden change in section that concentrates the bending and peeling at that point. Granted, you remove some wood when you scrape off the finish too, but I'd think it would be less, if you're careful, and with stress risers size really matters. On the few that I've seen where there was top material removed under the bridge, and the bridge subsequently came up, the tops tended to take a lot of damage.
I really like hot hide glue for putting down bridges. The reason may seem counterintuitive: it's brittle and has low peel strength. When a hide glued bridge does let go, it usually goes cleanly; the glue line fails, but there's little, if any, wood breakage. White (PVA) glue is the worst, the equivalent of bubblegum; it won't hold but it won't let go either. Yellow glues, such a Titebond, are better, but still kind of gummy. The bigger problem with them, though, is that new glue, even of the same type, won't stick as well as it will to bare wood. The bridge came up once with the best glue line you could manage with that material, why do you expect it to stay put with a less good glue line? The only way to get al the glue off is to scrape it off, and then you're likely to cut below the surface of the top, raising the risk next time. The sign of a bad policy is that it leaves you with no good choices.
I'm not trying to put down anybody's methods for gluing down bridges. I haven't tried everything. I'm sure we all try to use the best methods we can come up with. I'm just reporting on what I think I've learned from years of repairing all sorts of guitars. I believe that's the best school around for learning about what doesn't work, and thus improving your own practices.
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