Gap filling

[Jack Lord]

What is best for filling small gaps between rosewood binding and a spruce top?  Dan Erlewine's  book on finishing says "acrylic wood dough".  I can't find brand name for this.  What is it? These are very small gaps on a new guitar that want to fill before scraping, sanding and finishing.  I've read that super glue can discolor spruce.  The top has several coats of shellac.

[Andrew]

Yesterday I made up a patch consisting of spruce, ebony/rosewood/ebony purfling, and birdseye maple binding to be glued in place all at once. I laminated the whole piece together using the Hot Stuff brand 'Special T' thick viscosity cyanoacrylate and accelerator, and that didnt discolor the spruce at all despite that I was gluing up to the end grain. So that may be a good bet. Of course you dont want the accelerator in this application, but I dont think that played a big part preventing the discolouring, mostly the thickness of the glue rather. Of course it would be a good idea to test on a scrap piece thats finished the same as your guitar, or a rough equivalent. The Special T should give you just enough time to work it into the voids a little with a fine brush, which should also reduce the possibility of bubbles.

[Glen H]

Jack, I when I get a gap in your situation, I want to hide the gap, not draw it out, so I use something nearly clear like clear lacquer burn in stick, or system three expoxy. Next choice would be tinted putty to match the rosewood. Last choice would be something light colored. This all assumes it is a minor gap not wide. If it is wide I route off the binding and start over or heat it up and try closing the gap more with a clamp. I'm not familiar with dough.
Andrew, you might want to add the word "yet" after you found spruce didn't discolor with cyano. I've finished guitars to only find discolored spruce months later where I used cyano but did not seal the end grain well. Seems to bleed right into the wood under the finish over time. Your luck might beat mine though.

[Andrew]

Thats a very good point. And I dont have to worry about how it looks under finish in my case because the patch is under the fingerboard extension, with only the teeniest sliver visible at the edge. However, I dont see how cured cyano could continue to bleed into the wood. Perhaps it changed the way the wood reacted to the effects of light? But that's beside the point maybe. Regardless of what he uses Jack would be wise to do test pieces if unsure.

Anyway, heres a pic of my leftover piece that has been wet out with mineral spirits. I couldnt see any change in the wood where the glue is - with thin and sometimes medium cyano, the wood would 'darken' where it absorbed the mineral spirits, and appear lighter where the superglue was absorbed because that wood can no longer soak up the mineral spirits.

Not saying Im right or wrong or that there is no better method, but I think it's a viable option. That said, some clear burn-in lacquer would probably work well.

[Jack Lord]

Thank you both for your help.  Since I'll be finishing it with nitro lacquer and since the gaps are very small, what about filling it with nitro lacquer,  or wiil that sink when the final finish is applied?

[Andrew]

Nitro will shrink, and even when you think you have it filled, it will continue to shrink long after youve buffed it out and the gaps will show again. Another option is to shoot a couple coats of lacquer, see where it shrinks, and fill that with medium super glue. By then the exposed end grain will be sufficiently sealed by the lacquer and the glue wont discolour the wood. When the glue is hard and the finish dry enough, scrape away the excess and continue your finishing process.

I know, I keep harping about super glue, but it really is one of the easiest options for small finish fills like this and rarely causes me problems. You just have to take the characteristics of the glue into account and adjust for the situation (sometimes that does mean not using it despite temtpation). Obviously you should choose what works best for you.

[Glen H]

That's what the lacquer stick is but works a lot better than liquid nitro because it is solid. With thin gaps you can build up using thickened liquid lacquer but it takes longer and does tend to shrink back later. If you do use nitro thicken it first by leaving a small amount out for a few hours. Apply it then allow a few days for shrink back. Repeat this over and over until you get no shrinkback and then leave it alone for a couple weeks to confirm. Or just fill with some expoxy a couple times and you're done.

[Hesh Breakstone]

I've had very good luck doing the following.

Let's say we have a slight gap where the binding meets the side wood.  Smear some Titebond original in the gap and then with 120 sandpaper sand in the direction that the gap runs and ONLY in the direction that the gap runs.  This tends to take wood dust from the two respective types of wood and stick it in the Titebond and since we are keeping the direction the very same as the gap runs the two dissimilar woods tend to not mix and instead embed themselves in the glue closest to where the sanding dust was created.  This preserves the sharp contrast instead of blending two wood and only take 30 seconds to do.  Deeper gaps may require doing it twice.

Mind you this is for very slight gaps only and I've never experienced any issues doing it this way including after finishing, I can never find the gap again.

Never had any luck at all with fillers so I prefer real wood for a large gap and real wood dust for fine gaps and of course this need not be said but I'll say it anyway, building gapless is the best choice of all.

[Mark McLean]

Before filling the gap, see if you can close the gap by heating that section of binding to soften the glue (assuming that you used something like Titebond or HHG to glue the bindings on) and then pull it in with tape.  If that doesn't get it done, then I find that wood dust is the best filler.  I usually use dust from the binding wood - but the trick Hesh just described sounds like it could be a good way to go.  "Build gapless" - is a nice aspiration but I haven't achieved it in the six instruments I have made so far. 

[Hesh Breakstone]

Hi ya Mark!

A couple of things that really helped me get gapless with binding are as follows:

1)  Early on my bindings were tall and thick, likely near 1/4" tall and .090" thick.  In time I started to like the look of both shorter and thinner bindings so I went closer to .060" thick and shorter and discovered that they were way easier to snug up in the channels.

2)  Relieving the inner edge of the bindings (a couple of scrapes) that has to mate will with the channel inner edge AND doing a quick clean-up of the channel's inner corner helped things seat better.

3)  I use a Williams binding jig and the thing works great at keeping my channels square and uniform.

4)  Prebending the bindings in a Fox bender and trial fitting dry making sure that with minimal clampage the bindings seat well.

When I started doing all of the above things got gapless.

Hope this helps.