Bridge sanding technique

[natevw]

I just worked through Rob's great ebook (https://www.mandolincafe.com/forum/s...by-Rob-Meldrum) and it was super helpful.

I did end up deviating significantly from his instructions in one regard when fitting the bridge. He encourages feeling for contact with a sheet of paper, marking, and then sanding from that point "manually" — off the mandolin — and then sort of rinse/repeat.

For me things were going poorly trying to adjust the feet that way. While I'd get one side right the other would go off without hardly touchng it. Or would be focusing on fit from the tail side and realize the neck side was off kilter.

Finally I resorted to a trick I'd seen a violin maker use in a YouTube video, and that did me no worse. I see someone else has described this technique on the forums here (https://www.mandolincafe.com/forum/s...#post1680381):

[...] place a fresh sheet of sandpaper face up on the top, put the bridge back on, put some tension back on the strings, and work the bridge back and forth across the sandpaper.

Any downsides to this approach?

[Roger Moss]

Check out Jerry Rosa's videos on YouTube (Rosa String Works). That's the way he does it so it must be fine.

[Walt]

Any downsides to this approach?

I think this method is pretty common. I've seen discussions disagreeing as to whether the motion should be forward to back (fingerboard to tailpiece) or side to side (f-hole to f-hole). In my limited experience, I got good results going forward to back. I think the most important thing is to constantly check your progress. As with most thing, it's not as easy as just going through the motions. You need to make sure the bridge stays at the proper angle. Also, it's easy to inadvertently put too much pressure on one side of the bridge, so you end up removing material at a faster rate on one side.

[mtucker]

I think this method is pretty common. I've seen discussions disagreeing as to whether the motion should be forward to back (fingerboard to tailpiece) or side to side (f-hole to f-hole). In my limited experience, I got good results going forward to back.

Appears from the photo that this guy is forward to back also.

[HoGo]

If your arching is evenly curved across top under bridge and 1/2" in either direction you can rub across and be OK (90% of cases). It is more likely that the arch is consistent along the instrument axis for 1/4" up and down so this works universally unless the top arch is really weird (work of amateur or severely damaged or deformed top). I sand across the arch freehand all the time holding the base by the posts with thumb and middle finger pulling them towards each other (causing the base to bend a little) and push down with index at he center. I also press the sandpaper against arch with thumb and middle finger of my left so I press the edges of f holes down a bit (so the ends of the bridge won't be cut too much and also they work as a stop in each direction - I cut no more than 1/4" in each direction). If there is more wood to remove I scrape the roughened areas away with knife or scraper. I want the base have slightly more pronounced arch than top (1/64" or so gap under center of one footer bridge with no tension) so the ends seat perfectly with just moderate finger pressure over top of the bridge.
I just fitted bridge to newish KM900 yesterday this way and it went smooth like in most cases.

[Marty Jacobson]

Be careful. The abrasive granules come off the sheet as you use it. Plus there are almost certainly some pieces of the granules on the backside of the sheet. As you do the sanding a single granule can skip around underneath the sheet of sandpaper resulting in a multitude of scratches. Ask me how I know. :-)

[MontanaMatt]

Bruce Weber recently fit my bridge with the on the body sanding technique, neck to tail. Using a pencil to shade the feet and looking for high spots during sanding.

[rcc56]

Be careful. The abrasive granules come off the sheet as you use it. Plus there are almost certainly some pieces of the granules on the backside of the sheet. As you do the sanding a single granule can skip around underneath the sheet of sandpaper resulting in a multitude of scratches. Ask me how I know. :-)

Yeah, I posted the same thing a few minutes ago on a similar thread. There are two discussions going on about this right now.

If you use the jig, place a thin sheet of paper between the sandpaper and the top, and a thicker sheet between the jig's wheel and the top to avoid collateral damage.

[HoGo]

Yeah, I posted the same thing a few minutes ago on a similar thread. There are two discussions going on about this right now.

If you use the jig, place a thin sheet of paper between the sandpaper and the top, and a thicker sheet between the jig's wheel and the top to avoid collateral damage.

Of course this is important as well. I mostly do without paper between sandpaper and top (on harder old finishes) but I use high quality sandpaper that doesn't lose grit and I dust it and the top perfectly clean before each series of strokes (10-15). I've used the same dedicated piece of paper (actually two of them 100 and 180 grit) for over 50 bridges - I don't sand hard it works better with moderate pressure. If there's need to remove more wood, then scraper does the work. The light color of my paper shows where it sands ebony and scratch patern on bridge shows as well. I smooth the underside with scraper before each set of strokes on sandpaper.

[Ivan Kelsall]

Marty - When i fitted a CA bridge to my first Lebeda mandolin,getting abrasive material under the paper was one of my first thoughts.
I used a sheet of writing paper fastened to the top with low-tack masking tape,& then fastened a strip of wet & dry abrasive paper to that. The addition of .003" to the arch top didn't prevent me from getting an excellent fit to the top. I also sanded the base from side to side to perevent any chance of rounding off the front & back edges. That way,you don't need any type of jig - just a steady hand,
Ivan