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Top Finish Crack Repair

Posted: Mon Aug 27, 2012 10:52 pm
by Philip Secrist
I have performed many finish touch ups with great success, but this one makes me a little nervous. I have a Martin OM-28 on my bench with a simple top crack. It was a very tight crack. It actually did not crack all the way through the top. The crack has been repaired successfully by working in some glue and clamping it up. The crack closed up nicely but the finish crack is still visible and the unevenness can be felt when running the fingertips over it (the crack is visible in the reflected light area in the finish crack pic attached). The customer would like to sell it so he would like it to be as close to new as possible. The guitar is about 7 years old so the nitro finish has sunk into the top grain just slightly enough that you can feel a subtle ripple on most of the top surface. My concern is if I fill the crack with a tiny bead of lacquer, let it cure, level it and then buff it out, will the repair area stand out too much from the surrounding finish area. My plan, of course, would be to keep all leveling and buffing restricted to a very narrow area on either side of the crack as much as possible. Those of you who have done a lot of finish repair work, is it better to have a small smooth area around the crack compared to the slightly rippled finish on the other areas of the top, or to leave a visible finish crack that can be felt? Is there a compromise or a method that I am not aware of to blend this in and with the most success?

Re: Top Finish Crack Repair

Posted: Tue Aug 28, 2012 2:10 am
by Michael Lewis
You have already used up your one chance to get it as good as possible, so give us some information. What kind of glue did you use? Did you apply any cleats inside?

Do NOT rub fingers across the crack (I know, it's already to late) as that deposits dirt into any crevice.

Finish touch up depends on your skills. Remember that is is quite easy to make the crack much more apparent. If you are not confident you can improve the appearance then leave it alone, as it doesn't look too bad now. Do NOT sand or scrape the uneven surfaces level. It would be better to soften the glue and clamp the crack again to flatten it. It's a lot easier the first time round.

In the future use hot hide glue for things like this as it shows less than other glues. Also use flat plates for clamping cauls and waxed paper as a release for the cauls, and clamp till well dry, over night at the very least, 24 hours is better. I often use plexiglass scraps for the flat cauls. Rare earth magnets or a deep reach clamp to reach the crack and clamp firmly, and immediately clean up any glue or it can damage the finish if allowed to dry. A very thin smear of glue on the finish is not much to worry about as it can be cleaned up with a wet cloth later but DO NOT allow glue thicker than a piece of paper to dry on the surface.

Re: Top Finish Crack Repair

Posted: Tue Aug 28, 2012 10:27 am
by Philip Secrist
The crack ran from the bridge to the end of the guitar. First, I used de-ionized water and a new fine tipped brush to clean the crack. Then I used titebond glue, working it into the crack as I pushed on the crack from the inside. The crack didn't not go all the way through the spruce BTW so I did not install any cleats on the inside. I examined it carefully with light and mirrors on the inside prior to doing anything. After working in the glue, I wiped off the excess with a soft clean damp cloth. I then put on two clamps. One (with padded cauls) to apply pressure sideways on the lower bout. I also placed a flat plexiglass plate on the top of the crack and one on the back and clamped the crack to hold it flat. I let this dry for 24 hours. I rubbed my fingertip over it after the glue had dried the next day. I considered using hide glue (and should have apparently since I can't no soften the glue and try again), but I was concerned that I would not be able to work the glue into the long crack and clamp it up quick enough before it gel'd up. So, in your opinion, there is nothing else I can do? The remaining crack indication is subtle and feels more like a slight depression than anything.

Re: Top Finish Crack Repair

Posted: Tue Aug 28, 2012 11:43 am
by Barry Daniels
You probably caused the depression and the mis-alignment when you put a clamp from the top to the back. This pressed the top down into a slight v shape and caused the crack to be mis-aligned. The crack should have been clamped through the soundhole with a deep reach clamp or with magnets, like Michael described, which would keep the two sides of the crack even.

Re: Top Finish Crack Repair

Posted: Tue Aug 28, 2012 3:05 pm
by Philip Secrist
The crack was very tight to begin with and had a slight finish depression before I glued and clamped it. The crack didn't go all the way through the top. In fact I saw very little squeeze out when clamped so I am not sure it was very deep. But it does not open at all when pushing from inside now. It really looks a lot like it did before gluing except that the crack is even tighter and is less apparent. I didn't clamp that hard, but yes, in hind sight, I wish I had clamped through the sound hole instead of the outside. So is there nothing that can be done at this point to make the finish depression less apparent? (which by the way is very subtle and really not too different from the adjacent finish ripples but can be seen up close when held just the right way).