I built a dreadnought guitar with a three piece back out of beautiful Macassar Ebony with sap wood. There are 3 long hairline cracks that have opened up in the sapwood area of the back. (See the attached pics). They showed up after I had sprayed the guitar and let it cure for a few weeks. I thought that they were just grain lines that had not been filled entirely with pore filler and that the nitro finish has just sunk in. I sanded and level them out but they reappeared again after a week or two, so I leveled and buffed again and then gave it the customer.
I got the guitar back after about 9 months for a checkup and lines have appeared again. After sanding with 1000 grit to see if I could level the finish, I can now see that they are cracks. I can't get the cracks to move at all when I push on them, so I am not sure if they go all the way through the wood, but they definitely have opened up the finish and I can feel them with my finger.
Two Questions: 1) How do you recommend I repair these cracks? 2) I am not confident that I have enough finish there to level sand after the repair, since I leveled the area three times before thinking the finish was just shrinking into the depressions (dang it!). So, If I end up sanding down to the wood after the repair and need to re-spray the area to build up the finish and cover the repair, how do you recommend I do it so that I don't get over-spray on the rest of the guitar or leave lines where I mask the sides and top? Thanks for you help!
What I was saying is somewhat moot, Franks epoxy soak seems like consensus - but the same issue arises. The epoxy or thin CA will seep through the cracks and drip into the box so you can either tape up under the noticeable cracks, or put some filling inside to catch the drips. Taping up is a problem if the glue attacks the tape and sticks the whole lot together so use brown plastic packing tape which tends to not let either epoxy or CA stick to it even with the adhesive side to the glue.
As far as softening the finish before removal with a solvent or retarder - I'm hesitant as I have in the past managed to get it on the "good stuff" that I didn't want to remove - acetone used to damp a terry cotton cloth works on nitro. - which is the same as using thinners. I use heat gun for most solid body nitro removal but that's not an option her with the thin unstable back we are talking about.
Tell us how you went with this, it's a good learner for me.
If the crack doesn't go all the way through, you could put a jack inside under the crack to force it open, You could then put CA glue in it and release the jack. Excess glue will be forced out. Wipe with cloth damped with acetone.
It's entirely possible that there are other cracks in the back (I think I see one above the knot in your photo #173) that are not so obvious at this point, so it might well pay to presume that such things are there but not so obvious at this point. Some builders I know will respond to this problem by scraping and sanding the entire finish off the back, saturating the whole surface first with a good grade of slow set epoxy like West Marine and sanding it off after it sets, then doing the same with thin cyanoacrylate.
Believe it or not, epoxy will penetrate the tightest cracks as well or better than the thin CA, but doing both can be a bit of insurance against future surprises. Then the application a new finish may give you a fighting chance. . .
Thanks Frank! Yes, you are absolutely correct? There is a 1.5 in. crack above the knot. It is a beautiful guitar that I want to give the best chance of long term survival! A few questions, (1) I do have SB-112 System Three epoxy. Is this a good epoxy to use or do recommend something better? 2) The back is somewhere between 0.070" and 0.080 inches thick. I certainly want to minimize wood removal during the process. Any recommendations on the best method to remove the finish and minimize wood removal? 3) How do I best level out the ridge that will occur on the binding edge where the new finish meets the old finish? I assume I will need to mask the top, sides and neck right up to the rounded edge of the back binding. 4) I would probably use a plastic garbage bag to mask it off. Is this the best method. Sorry for so many questions. Thank you in advance for your advice on this project! :)
System Three is good stuff, I believe, and I think the SB-112 is probably similar to the West System stuff I use. You do want to stay away from the quick hardening "Turbo" agent, so the epoxy will have time to seep into the cracks. If it seems too thick, you could add a bit of acetone to thin out the mixture before application, but that will add some drying time to the proposition. Yeah, masking is always a good idea, but don't leave the tape on long - remask if necessary. Scraping off the partially cured epoxy might help keep the sanding to a minimum, but bear in mind this is all unpredictable stuff - shrinkage can continue, other cracks can show themselves, etc.
Thanks Frank. Any recommendations on nitrocellulose lacquer softener so I can scrape it off? I realize there are no guarantees with any process. :) Thanks again!
I'd stay away from solvents at this point, for fear of driving more contamination into cracks. I'd scrape and sand carefully dry, blow of the dust as well as I could, then proceed with the CA and epoxy, doing the same with each layer of that stuff, too. Nasty work, for sure.
Thanks Frank. I was looking for an easier way to do this and may have gotten myself into more trouble. :( I tried just filling the cracks with thin CA from the finish side and cleated in the inside. I scraped the repairs flush with the finish with a razor blade with scotch tape on the sides (or though I had sufficiently) and sanded the entire back level with 220 grit, and then resprayed the back. I couldn't see the CA around the cracks or feel the repair before I sprayed, but here is what I saw after spraying one coat of nitro on the back. See pic. It appears that the new finish did not flow over the crack repairs and conglomerated on the edges of where the CA was applied (which obviously didn't get scraped down enough). So, I dropped filled the crack several times over a day and then scraped the area flush the next day and re-sanded the back to get it all level, and shot the back again with another coat and.... I still see the same thing. The finish is not flowing over the repairs. So, now I am wondering how to proceed. Could I sand the entire back down just enough to remove the excess CA around the cracks, or do I need to go all the way to the wood and do as you first suggested? Thanks Frank.
Hey Frank, I have scraped and sanded off the finish on the back down to the bare wood. I would like to apply epoxy to give the back a fighting chance. When mixed, my system III epoxy is of honey consistency. How much do you think I should thin it with acetone to give it the best chance of penetrating into any unseen cracks?
I second Robbie's suggestion. Sand it back and shoot a coat of Zinsser SealCoat shellac.
Thanks Robbie. I have scraped and sanded the finish back to bare wood. I am going to try the epoxy method Frank suggested to fill any other unseen cracks and give the back a "fighting chance". After doing this, would you recommend spraying a coat or two of fresh shellac before starting with the nitro top coats? Thanks!