Filed my nut slot too low.. :(

[HodgdonExtreme]

Thank you all for the replies. Today I ordered some UV-curing resin that is supposed to set "very hard". I've got a UV-C disinfecting wand/light/tube to cure it with. Supposed to be here Thursday so I'll give that a shot and see how it fares.

Else, I'll buy some fancy CA glue instead of the basic "superglue" brand that comes in a 5 pack of mini tubes, and give that a try.

I'll keep y'all posted!

And thanks, Charles, for the explanation of nut-slot geometry from a sideview perspective. I understand exactly what you mean. I'll start doing it that way in the future!

[HodgdonExtreme]

So my UV-cure resin arrived and I tested it out to see what consistency it would cure to.

It was advertised as "extra hard" and "glass-like".

Not so much. I tried curing out in direct sunlight, as well as with my 11 watt UV-C lamp, and it cures to a stiff, but pliable texture.

So I sent that back and I'm awaiting a different brand that also cures "super-hard" and "glass-like". Fingers crossed!

In the meantime, I've been playing the guitar with the buzzy/twangy/sitar-like G string. Frankly, I prefer this to the tuning instability and hysteresis problem I had before.

More to follow...

[HodgdonExtreme]

Received new batch of UV-cure resin, tested its cured consistency/hardness. Also pliable and not suitable as "nut material".

A friend has a Bondiq UV kit, tested it - again, not impressed with how it cures up.

That's when I got the idea to buy dentist/cavity filler resin. It's a lot more expensive per gram than arts/crafts resin, but oughtta cure into a tooth-like hardness/texture.

A syringe big enough to probably repair 10 nuts is available on ebay/amazon for about $10, and you can order it in different shades of white/yellow to match your nut! You can print out a shade-guide to hold next to the nut to choose the correct shade.

Haven't received it yet, but I'm hoping this will prove to be a good plan.

More to follow

[mtdmind]

Is it possible to just get a new nut installed by a repair person?

[YamahaGuy]

As much time/$ as you're spending on the workaround, a new bone nut could be procured, filed, sanded, installed. Every guitar I've owned has gotten a new nut. They're not that difficult to change.

[charles Tauber]

Originally Posted by HodgdonExtreme

That's when I got the idea to buy dentist/cavity filler resin.

I've read of someone using that successfully.

The industry "standard" for filling nut slots is bone dust or baking soda with CA glue. There are other, possibly better, methods, but that is what is most often used. I've successfully used that for two decades or so with no incident and no premature wear of the filled and recut nut slots.

If you are having problems with the CA method, I'd look to your materials or technique. There is nothing wrong with finding a better mouse trap, such as UV-cured dental materials, but it isn't necessary.

[RoyBoy]

If you're already in an overwide slot and the fill feels "grabby" then you likely need better/fresh CA glue. I've taken to using Starbond with the microtips. Since I have scraps of bone nut stock on hand I make bonedust with 150 grit paper and use that for filler. You must use good tape (I like the green stuff) and mask off carefully around the slot.

Personally, I think 0.032" is wide for a G string slot, definitely not 0.036". I take the file and lean it over 3-5 degrees to one side and then 3-5 degrees to the other side to make a lightly wider slot. Like Mr. Tauber, the bottom of my slots arc downward similarly to the curve you drew for the top of the nut. The angle where your ledge meets the down slope will bind on a wound string when loosening tension.

Since you've patched this slot a couple of times you may need to get into fresh bone to get a good solid slot. That would put your options at shimming the whole nut and deepening all the slots to optimal depth or getting a whole new nut (which may require enlisting a good luthier). I would start with quality CA first and see if you can salvage the existing slot. Good luck!

[Robin, Wales]

I had to fill a G string slot a few days ago and re-cut it. I used bone dust from filings off an old nut and very low viscosity superglue. I use the cyanoacrylate that is the consistency of water and goes off in <3 seconds. I buy the smallest bottles I can find and a pack of micro applicator nozzles. Keep the bottles in the dark and it will stay fresh longer. Superglue works really well on bone!

[Howard Klepper]

Any glue or resin fill is a temporary fix that will tend to be grabby. The easiest way to get a permanent fix that will make it functionally perfect is to inlay a piece of bone, about 1/8" square, glued into the nut with superglue. Shape it down and recut the slot. Get a tight fit and it will also be nearly invisible.

[Robin, Wales]

Originally Posted by Howard Klepper

Any glue or resin fill is a temporary fix that will tend to be grabby. The easiest way to get a permanent fix that will make it functionally perfect is to inlay a piece of bone, about 1/8" square, glued into the nut with superglue. Shape it down and recut the slot. Get a tight fit and it will also be nearly invisible.

That sounds a lot simpler than I expect that I would find it in practice. I think that I would struggle to cut out a nut section, then shape a piece of bone fragment to fit the gap.

A more simplistic solution I have used is to shim the nut if the glue fill method does not work for some reason.

I have an envelope full of thin marquetry veneer offcuts. I will glue the nut onto a piece, trim it, refit the nut and re-depth all the slots.

[LiggettGuitars]

Originally Posted by Robin, Wales

That sounds a lot simpler than I expect that I would find it in practice. I think that I would struggle to cut out a nut section, then shape a piece of bone fragment to fit the gap.

A more simplistic solution I have used is to shim the nut if the glue fill method does not work for some reason.

I have an envelope full of thin marquetry veneer offcuts. I will glue the nut onto a piece, trim it, refit the nut and re-depth all the slots.

In my opinion, as with most luthiery, if you go too far, you should redo it. I feel like luthiery as a whole is a game of "don't go too far" or being right on the verge of not messing something up royally. Stewmac has some guard contraption that'll keep you from going too far if it's a recurring issue.

The super glue/dust or even resin/dust combo has always seemed gummy feeling to me in comparison with bone, and I'd feel terrible charging someone to make a nut only to have their slot wear 10 times faster than the others from tuning friction and get a buzzy string in less than a year. (I've tried this on personal instruments, I don't think it's a great permanent solution)

Ben

[HodgdonExtreme]

Update: My nut slot repair using dental cavity resin seems to have been a resounding success.

I wanted to wait a week or so before providing an after-action report, to be sure it was doing what it's supposed to...

I used Dentex DX-Flow "nano flowable light cure composite" material in the "A2" shade - which matches the bone nut on my Martin absolutely perfectly.

The stuff comes in a tiny syringe filled with 2 grams of the substance. It's probably enough to do a repair like this 100 times. It has a shelf life of 5 years. It is about the consistency of natural peanut butter, so you can work it down into the nut slot easily, but it stays put, and doesn't flow out. You have total control of when it cures, so you can do your cleanup BEFOREHAND, so that you are ONLY getting the resin exactly where you want it. A huge advantage compared to CA glue.

I simply dispensed a tiny little blob out onto a piece of plastic, and used a toothpick to pick a little bit up and work it into the slot. Very easy.

It cures to a very, very hard texture - kinda like your teeth. My StewMac nut file was able to cut it, but it wasn't easy. Where as bone files away readily, making it easy to go "one stroke too far", this stuff takes a bit of work to file, so the chances of going too far are really low. My file doesn't seem to have been damaged by cutting this harder material.

After the 20 second cure time, It took 10minutes to get the slot filed like I wanted it (stopping to check/measure frequently) and then another 3-4 minutes of sanding with 320 grit, 600grit and 1500grit sandpaper to have a flawless repair.

Functionality-wise, it works perfect. The "hysteresis" and tuning instability I wrote about earlier is gone, and of course, so is the dreaded buzz from the slot being cut too deep.

I found this approach to be much easier than dust/Baking Soda and CA glue, because you can absolutely ensure you are only getting the stuff exactly where you want it; it just wipes away until you cure it under the special light. I also think it was much, much easier to perform than fitting a new nut or shimming the old one.

Cost was $10 for the resin, and $25 for the special curing light.

Hope this helps someone in the future!

[mtdmind]

I am glad this worked out for you inexpensively. Plus, you learned lots! Thanks for the information.