Defretting and the use of Wood Filler

[Turnaround]

There has been a recent discussion in the Hardware Setup And Repair Forum about the suitability of wood filler to fill fret slots when defretting a bass. At the risk of being redundant, I thought it might be useful to reiterate here.

Conventional wisdom amongst most (if not all) of the repair professionals I know is that wood filler is not a suitable material for this purpose. The concern as I understand it is that it compresses too easily and consequently would affect the stability of the neck under tension. I am not aware of any tests done that would support this view, so at the prompting of a contributor in the aforementioned forum, I took it upon myself to perform a simple test. Bear in mind that the test I performed was not a "scientific" one, in that I have not the means to do so, but I will outline what I did and share the results.

I glued a piece of veneer roughly the size of a fret slot along one edge of two blocks of maple. Along the other edge I packed in the same thickness and depth of a popular wood filler:

Crush Test Setup

• Turnaround
• Feb 23, 2015

I allowed the filler to dry for 24 hours, then dressed the assembly to the same thickness throughout. I then applied pressure to the assembly for a period of one-half hour. I then measured the thickness of the assembly long the veneered edge and the wood-filler edge. The veneer edge showed less than 1 thousandth of an inch of compression (maybe none since my gauge can't measure that finely). The filler edge showed compression of about 4 thousandths of an inch, and was fractured.

As I pointed out in the other thread, we need to be careful what conclusions we draw. It could be that I had a bad batch of filler. It could be that the forces I applied were no where near what would be experienced in an actual fret slot under string tension - maybe less, maybe more. All we can say is that under the conditions I applied the wood filler compressed a noticeable amount and the veneer did not.

I have fixed numerous basses over the years that were filled with wood filler in the slots. The ones I fixed were exhibiting excessive forward bow that required a lot of tightening of the truss rod to overcome. The fix was to remove the filler and replace it with close fitting, glued in veneer. That corrected the problem and required les truss rod tension to hold the neck in proper relief under string tension. I have never had to replace the veneer. That is not to say that the problems of forward bow in those cases was definitely caused by the crushing of the wood filler, but I was operating on "conventional wisdom" at the time.

There are many who have reported good success using wood filler, so we cannot say that using it will lead to problems. But I believe collective experience indicates that it can apparently do so.

I need to be clear about the wood filler I used. I am not going to name the brand since I don't want the argument to degrade into brand wars. Suffice to say it is a readily available and highly recognized product, certainly one of the most popular in my area. I did not use the wood filler you can get in a squeeze tube because apparently they are mostly latex-based, and I really doubt that they would offer much in the way of rigidity. Nor did I use the two-part wood fillers since I think they are a form of epoxy, and that's not what I see most people using as "wood filler" for fret slots. Epoxy is a whole other discussion, and maybe we will get to that.

I am particularly interested to hear the response from other members of the Pro Bench on this topic. Do you agree with the conventional wisdom against using wood filler? What has been your experience? What is your preferred method of filling fret slots? Have you done some test? We may not be able to put this to bed, but perhaps my test will add some weight to collective experience. Or perhaps our collective experiences aren't in agreement.

 

[mongo2]

I prefer glued veneer.

 

[Bruce Johnson]

Yes, I agree. Most of the non-epoxy wood filler pastes are weak in compression strength. Push on them, and they squash down and squeeze out the sides. Worse yet, they have very poor vibration resistance. Hit them with a pulsing load and they shatter and turn to powder. The inside of a fret slot in a bass sees a pretty serious pulsing compressive load. Like, every time you pluck a string.

Epoxy-based wood fillers are better, but most of them use a fairly soft gooey epoxy. It will tend to creep over time. Better yet is a hard marine epoxy, like West Systems or System 3.

My three recommendations for filling fret slots:

If you want to make the slots invisible (well, nearly), in an ebony fingerboard, fill them with black-tinted West Systems epoxy. Make masking tape dams along the sides, feed the epoxy into the slots, let it cure. Sand it level with the fingerboard surface and polish it. It will last decades without shrinking or cracking, and you won't hear any perceptible sound difference as you slide over it. I've used West Systems like this for filling slots, and for removing inlays and dots. The black epoxy fills aren't perfectly invisible, but they are pretty close.

Like Richard says, wood veneers. Thin strips of contrasting or matching wood, fitted tightly and glued in solidly. Simple and stable. It's best to use a wood that's roughly the same hardness.

Styrene plastic also works very well, particularly for white lines on ebony or black lines on maple. This is what I usually use for fretless conversions. I get white styrene strips at the hobby shop, which are 0.020" thick x 1/4" wide. Glue them in, trim them with a chisel and a file. Simple and durable, no sound difference as you slide over it. The plastic is just soft enough that it will tend to wear with the wood, if the wood shrinks. It doesn't get sharp edges. Plus, it's easy to saw the styrene out, if you later want to go back to frets.
​

Don't be tempted to use metal! Been there, done that, pulled them back out. Brass or aluminum will end up sticking up slightly, with razor sharp edges. Ouch. And you'll hear a distinct sound as you slide over it.

I should also mention that Pulsing Compressive Load would be an excellent name for a rock band.

 

[Turnaround]

Hmmm... not sure about epoxy. Though there are those that advocate the use of epoxy in many areas of instrument work, there seems to be reluctance amongst some luthiers about its use. Primarily the concern is about "creep", that is epoxy's tendency to distort over time under stress. There have been many studies about this creep behaviour, and there is no doubt that it is real. So I am not sure I would recommend its use in fret slots.

Though there are a number of different epoxies out there, it is my understanding that they all exhibit some degree of creep under stress.

 

[Bruce Johnson]

Richard;
Most hardware store epoxies are too soft, and will creep. However, hard marine epoxies like West Systems don't creep at all. I've been using West Systems for 20+ years for structural joints, filling gaps in repairs, and filling grain under finishes. It doesn't sink or creep. I even did a long term durability test a few years back. I left two bodies hanging outside in the So Cal sun for 26 months. The West Systems never shrank, creeped or cracked.

I understand that System 3 epoxy is similar to West Systems, although I don't have any personal experience with it.

 

[Turnaround]

There are so many studies out there that show that epoxies creep. Apparently the more cross-linked the epoxy is, the more it has a tendency to creep. But the indicators I have seen show that all epoxy creeps. Then again, glass creeps too so I am not sure I know how to interpret the scientific studies. There must be a critical level of stress where this creep becomes significant, but I don't know if that's within the stresses we are talking about with respect to instrument repair, nut slot filling, etc.

I do know that a tight fitting wood veneer has worked well over many years of use, as has styrene. I prefer the wood veneer because I like the idea of using organic materials in a musical instrument wherever possible (I prefer nuts made of bone for the same reason). But I won't be a pig about it. Perhaps I should repeat my test with some epoxy - would you take over some of my repair jobs so I can spend the time???

 

[sissy kathy]

Turnaround- sure I'll take over your repairs while you do the research - you may not like the hit your reputation takes as a result of that though.

 

[Turnaround]

Turnaround- sure I'll take over your repairs while you do the research - you may not like the hit your reputation takes as a result of that though.

Let me think about that .....

Thanks for the offer, but I'm getting some really good work at the moment. Don't want to miss it.

 

[walterw]

Then again, glass creeps too...

i think that was disproven, folks misinterpreting medieval cathedral window glass that was thicker at the bottom as viscosity, not noticing that those old glass panes were equally likely to be thicker at the top.

 

[walterw]

anyway, has anybody tried matching wood dust and CA? maybe even backbowing the neck a little beforehand to allow for some theoretical amount of fret-slot compression like the frets would have provided?

CA sets up pretty hard and brittle, and it strikes me that a slot filler of wood dust surrounded by a CA "binder" would be pretty rigid and non-compressible.

 

[Turnaround]

I have never done the CA and wood dust method. But CA is very rigid and sounds like a good idea. Might be especially useful on a neck with a bit of forward bow. Might just have to give it a try.

 

[Bruce Johnson]

Yes, CA is hard and rigid. I use sawdust and CA sometimes for small spot fills. I don't like it as much for filling larger gaps, like fret slots. It would be better to fill most of the volume of the slot with a slice of wood or styrene, gluing it in with CA. That works fine.

 

[walterw]

It would be better to fill most of the volume of the slot with a slice of wood or styrene, gluing it in with CA. That works fine.

sure, that's what i like too; i lean towards contrasting wood veneer, done right the result looks really cool.

 

[202dy]

As far as I can tell Don Teeter was one of the first to advocate, or at least write about, the “epoxy refret.” He and others were developing and using this in the seventies, possibly as early as the late sixties.

The technique included filing down, and eventually included removing, the barbs from the fret tang so that the frets could be “press fit” into the slots. The use of “press fit” here does not refer to the term as used by machinists. Sliding or drop-in fit might be a better description. When the barbs were completely removed there was not even a friction fit between the fret and the slot. The fret could easily be popped out of the slot. Some techs actually enlarged the slots so the tang would barely touch the walls of the slot. The slots were filled with epoxy and the frets inserted.

Various clamping methods were used. Teeter devised a tapered caul with rods let in then glued into place on the mating surface so as to contact the edge of the frets. Another caul was placed on the back of the neck and a clamp was applied. Others used blocks like the Stew Mac radius blocks the same way.

Once cured the frets were dressed normally.

Over the years, many techs found that these necks had an inordinate amount of flex. This was especially critical on Martins because they did not have an adjustable truss rod to counteract the forward bow. The epoxy refret technique fell out of favor because of these kinds of failures.

Some techs continued to epoxy frets but left the barbs on the tangs. The idea was to have the epoxy help hold the fret in. These necks did not, as a rule, exhibit the same tendencies as the original epoxy refret.

What does this mean? It’s hard to say as we do not know what kind of epoxy was being used. However, it can be assumed that in the seventies and eighties most techs were buying epoxy at the local hardware store rather than finding a source for a hard marine product. One thing we can probably take away here is that cheap epoxies flex and that is not good for fingerboards.

When the client wants the lines to be invisible (or nearly so) I use shop made or store bought veneer of the same species as the fingerboard. The veneer strips are glued with Titebond. For a lined board my preference is styrene because it looks the cleanest and does not appear to compress very much. However, it is up to the client to choose between styrene or a contrasting veneer. That means using maple on rosewood or ebony, or rosewood on a maple fingerboard.

I’ve never used filler in a fret slot. The first time I saw it was on a used bass guitar hanging in a friend’s store back in the late eighties. He asked me to do a set up on it. The neck had too much forward bow. While it was on the bench I scraped my thumb nail along the fill and found that some of the material came out of the slot. That was all it took to rule it out as a viable material for filling slots.

There may be a filler product, other than the epoxies that Bruce refers to, that would not compress very much under load. But ultimately, what is the advantage of using filler over using veneer? The difference in bench time is not significant, especially considering the cost of purchasing a small amount of veneer. There is little advantage in material cost, but not enough to be much of a discount to the client.

When removing frets I keep some toothpicks and a container of water thin CA on the bench. Even after heating the frets some chips are to be expected. The chips are glued down before moving to the next fret. While it is a bit time consuming the finished product looks like the lines were installed at the factory.

This seems to be a lost art. It is rarely, if ever, mentioned in any of the tutorials found on line. Unless the board is going to be sanded to the maximum chip depth the look is terrible. Nothing is uglier than tan filler sitting in the missing chip slots on a defretted neck.

 

[Turnaround]

I hate the ugliness of filler in the chip-out as well. It reeks of an amateur botch-job.

To help avoid chipping when removing frets, I first run a small bead of water along both edges of the fret before applying heat. To help the water lie along the fret edge I usually add a bit of detergent to the water - helps to break the surface tension. Then a hot soldering iron on the fret until the water steams liberally. This softens the wood a bit and reduces the chipping somewhat when the fret is extracted. I make sure I leave enough time for the wood to dry fully before proceeding. Any chips that do rise can be immediately secured with CA, no need to wait for the wood to fully dry. It's better to deal with the chips as soon as possible rather than risk losing them.

When sanding a fretboard I carefully collect the dust which I keep in small vials. You never know when you will need to make up a bit of filler for fingernail divots and the like.

I've never trusted epoxy for filling the fret slots, probably because of the number of times I have heard reports of "epoxy creep". And I don't recall ever applying epoxy as a top coat to fingerboards either. I don't care for the harshness of sound it imparts to the instrument. But that's a matter of my taste vs that of others. If a client wants epoxy on their fingerboard, I refer them to another tech. Just kind of rubs me the wrong way and I think the customer would be better served elsewhere. What I do, I do well. I don't think epoxy coating is one of those things.

 

[202dy]

Glad to know I'm not the only one who saves wood dust. Used pill containers are great for that. I label them and include a short description: Coffee wood, 1962 Gibson xxx, light. Seems ridiculous until you need it.

I put a piece of white card stock under the guitar when pulling the frets. It makes the chances finding the escapees greater and prevents them from getting lodged in the bench carpet where they will only be found by the shop vac.

How long do you let the water soak into the fingerboard before walking the frets out? Do you think it is steam that is helping reduce chipping or just the water softening the wood How long do you allow it to dry before filling the slots?

On a personal note, I prefer the sound of the wood, too.

 

[Turnaround]

I think it's the steam that's doing the trick. It's just enough to reduce the brittleness of the wood at the surface. I don't expect that the steam penetrates very deeply, so I just let it dry overnight.

I wish I could say I liked the sound of wood. I have a very pretty piece of maple in the shop that's been there for years. It has yet to make any discernible sound. I thought I heard the Tulipwood once, but I might be mistaken.

 

[202dy]

You need to get out of North America. I have a piece of cocobolo that won't shut up.

 

[sissy kathy]

I can't imagine that you all haven't already thought of this. but I've never heard anything about it so I'll throw this out there for you to tear to pieces.

In severe cases of neck bowing, either the back of the neck is stretching, the finger board is shrinking, or a combination of both is happening; so could you pull the frets and place shims in the slots AND then REPLACE the frets. Wouldn't this cause a neck to straighten or even have some back bow, and would it affect intonation to a significant degree? Further, if the answer to my question is favorable, could this same procedure be used to mitigate ski jumps?

 

[Turnaround]

Compression fretting is a technique whereby frets with slightly larger tangs are installed. The extra thickness of the tangs effectively "stretches" the fingerboard side and introduces a bit of back bow in the neck. This is a method that has been used for many years by repair folk to deal with guitars that didn't have an adjustable truss rod - e.g. earlier Martins.

The principle doesn't really work with a ski jump which is not the same as a gradual curve along the neck, but more pf a kink near the body joint. To fix a ski jump you would need to apply all of the force in a small area, and that's not going to be possible with one or two fret slots. Good observation on your part though.