Anyone have experience with light colored opaque epoxy as thin spruce filler?

I have a center seem separation too narrow for a splint. It can't be brought together or widened with humidity. It is above a very large and thick twelve string bridge plate. The instrument was clearly heated and dried enough to pull the seem apart, but now that its cooled, it's held solidly open by the plate.

It seems like an ideal situation to try using epoxy as filler. Anything translucent will read as a dirty crack from a certain angle and even the thinnest splint I can pull off I think will be much more difficult to hide.

Does anyone have any tips/ experiences color matching using light colored pigments in epoxy?

I will be clear coating lacquer over the top.

I see this situation with some regularity and want to come up with a reliable, cost effective way to deal with it consistently.


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From a furniture backround, the primary problem with this is the color shading in the wood. From one direction it might look great, but from another angle really stand out. Never tried it on a guitar, but spruce shades more than many woods.

Why Epoxy?

I'm open to other Ideas Ned

Epoxy won't shrink as it cures, penetrates small spaces very deeply, strong, can be colored, has some adhesion to the Titebond lined trough. 

It's this particular situation. A rigid separation too small to splint but large enough to appear black, tapers to nothing, and is lined with failed AR glue. The entire separation is firmly glued to a .200" thick rosewood bridge plate.

You will get some great solutions to your problem  from Members here David, but I'll bet none of them will involve epoxy for it.

You will get some great solutions to your problem  from Members here David, but I'll bet none of them will involve epoxy for it.

O.k. I'll settle for just a "great solution". :)

Epoxy is a perfect solution and I can relate to the turned up noses at the notion, I used to feel the very same way.

The technique as we use it was developed at Elderly and is as you suspect David, tinted epoxy.  It's used when the crack is too tight but not closed AND forcing it closed either is not happening or would encourage cracking somewhere else when fixed and cleated.

Dave Collins does all of ours for us and I have yet to try it but he's done many.  I'll vector him into the thread tomorrow when I see him for some tips.

What I can tell you since I often am the one handing over the instruments to the clients for their approval is that it works like a charm, is easy to do, can be nearly invisible, and does not fight permanent dimensional instability and create other cracks later.

Remember too that I'm the one who usually spells epoxy as ep*xy....  If I can change my mind .... well you know...;)

Wanted to add that Dave does the ep*xy... but I often do the prep and bjr work.

If the crack is dirty... wood bleach can improve the appearance and make it more uniform for the ep*xy match first.

We use s small, narrow, thin bjr (back joint reenforcement) segment the full length under the crack.  It's cleated open, as it is after being at 45%ish RH or normal RH for your area for 48 hours or more.  The idea is to not be forcing anything.

A bjr is simply cross grain spruce as we see connecting the two back halves.

Then the epoxy is applied and Dave can talk about how he does that.

Thanks Hesh. Any input you or Dave have is certainly welcome.

I usually use wood and hide glue for repairs but I do it because it’s frequently the best solution not because I’m dogmatic about it.

This crack, as with many others like it, is only. 0015”-.002” wide. At first glance it appears very dirty. A quick look though a microscope and a stream of air show it’s clean as a whistle. The dirty appearance is because, although it’s skinny, it’s a chasm the full thickness of the top and it’s darker than the presidential election down there.

I use splints in many situations but it’s easier when a crack is wider and not anchored firmly to the bridge plate. The plate precludes manipulating the width of the crack with moisture in the top.

I could dig out a wide enough slot for a splint, but I just can’t think of an advantage if an epoxy fill can look as good or better. Even if future generations decide this 80’s 12 string is worth $100,000 it could be argued an epoxy fill was less invasive than the alternatives.

I think any filler used to fill up a .120”+ deep crack must not shrink. Here again epoxy seems to fit the bill. (so to speak) Gravity will apparently take The System Three epoxy I use to the center of the earth so I'm pretty confident it will flow into this crack.  But, if it’s not opaque, light will go down in the hole and it will just look like a filthy crack. Even if it’s filled flush and has a matching gloss on top.

I don’t work with solid colors much so I have some powdered pigments on the way. They should be compatible with resin. Unlike a lot of the touch-up I have done, it doesn’t seem possible to sneak up on a good match. I’m hoping for some guidance on that aspect because visual success rides on it and I have to get it right on the first shot.

Again, If anyone has another way I’m willing to listen. 

Epoxy would be my first choice. Unless a crack is so wide that it needs a few grain lines, epoxy is my go-to preference for a lot of top crack fills these days. I typically use West System 105/205, the light fairing filler (colloidal silica or microbeads, can't recall). This thickening agent has a slightly tan color to begin with that makes for a nice starting point for an opaque spruce fill.

Mixol tints are great for this, as they are quite opaque, and their white seems to work quite effectively when you need to lighten it up or make the base more opaque. Then maybe some TransTint amber, often the Kamel (tannish-brown) Mixol tint, sometimes some powder tints I have around. Just don't go too dark. I always tend to start off too dark. 

I mix on the fly for these, often changing the mix and wiping it in to adjust as I go. the hardest part I find is often when you squeegee off the excess, there's always that one or two little pockets or gaps that come up, so you have to watch for those and refill as necessary. 

For tight cracks like this, I think epoxy fills are the best and most appropriate solution on all by the most rare and valuable pieces. Tight crack, recent model mass produced 12-string, epoxy fill is perfect durable repair, and you should be able to make this crack near invisible pretty easily. 

Thanks for taking the time David.

 The specific color product info is a big time saver.

A long time ago I inherited a sticky box of West system boat building cast offs. I remember a tub of the flesh colored flour/filler. Looks to be #410 micro-lite filler I'll track it down. Great tip.

I'm going to smash some cracks in a dead body I have and start the learning curve. 

West System 105 resin is clear and the 205 hardener is an Amber color, which lightens when mixed 5 to 1. Colloidal Silica, a thickening agent, is White and 105/205 mix becomes a Yellowish color and more opaque when the Colloidal Silica thickener has been added. I'm guessing, since David can't recall, the he is using something other than the Colloidal Silica for the mix to end up Tan. I have not used anything but Colloidal Silica, so I can't comment on the other additives.

I have not tried epoxy as crack filler , so I have no first hand experience with the method but but I'm not sure why you would even need a thickening agent. I would intuitively think that it would harder to get the material into the tiny crack.


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