Official Luthiers Forum!

Hey all,

I’ve done a bunch of electric guitar setups and some repair but never a body repair on a hollow body. My Cordoba classical has a crack on the front just inside the lower bout that I probably wouldn’t bother fixing because it doesn’t impact tone but, as an excuse to practice repair I’d like to give it a try.

I’ve attached a photo. It propagated from a ding that I think came from my wife getting pissed at me and throwing something at it. I think it was her because when I mentioned it had a crack she said “oh that’s weird” and looked at the ground like she knew full well it had been there for a little while and was hoping I’d never notice

I think I need some hide glue, and a strip that I stick inside the body along the crack. Can someone suggest a specific glue and backing strip (if needed) and how to brace the parts together during glue up? Alternatively if there is a specific video that’s good… I know there are bad ones and it’s better to have a knowledgeable person suggest one than have me guess.

Your suggestions are greatly appreciated. Thanks in advance!

No photo....

_________________
"Act your age, not your shoe size" - Prince

Get some naphtha and an eye dropper and drip it along the crack on the outside of the instrument with the guitar on its back. Get mirrors and a flash light and check out the inside where the crack is.

Do you see a wet spot from the naphtha migrating through the crack?

If not it's a finish crack and should be approached as a finish crack. If you do see a wet spot likely following where the crack is on the top then it's through the wood and a different approach is necessary.

This location is so close to the linings and possibly under the linings in some areas that reenforcement (inside) may not be necessary. But before we go there find out if it goes all the way though?

PS: Naphtha will not harm the finish so no worries and it evaporates on it's own.

_________________
Ann Arbor Guitars

I’ll do this. I have to buy some online, though, since the local shops don’t seem to carry naphtha anymore. I tried three hardware stores and paint shops and they all carry some paint stripper that they say is a naphtha substitute but it doesn’t say what’s in it.

I’m pretty sure this is a crack and not a finish issue based on how the body moves if slight pressure is applied, so if I can trouble you for a glue suggestion so I can order that with naphtha, I’d greatly appreciate it.

Naphtha is also sold as zippo lighter fluid and Coleman fuel, or at least it is over in here in Australia.


Sent from my iPhone using Tapatalk

We use mineral spirits pretty interchangeably with naphtha and it won't harm lacquer finishes either. But if you can pulse the crack with your fingers as you said (be sure to wash hands dirt gets carried by our glues into the crack and they a dark line is visible) if you can pulse the crack then you're right it's likely all the way though.

Titebond original is my choice HHG has no value and in fact may be problematic on this one. There is no need to have things clamped up in 15 seconds or so and Titebond Original does a great job on cracks.

Cracks that are more accessible the process is use RH to close the crack and if it will stay closed in semi-normal RH then glue it with Titebond original. We prefer TB over HHG for this because after the glue is in place and if it's a tight crack getting glue in the crack can be difficult with pulsing the crack and time required. We don't have any time with HHG.

There is also the last step on cracks that are more accessible and that is to overlay them with waxed paper and then a rigid caul that will conform to the top dome. Then rare earth magnets, not the ones you see here but the ones that can send you to the ER with 42 lbs per inch of clamping pressure are used with the calls to ensure that the glue and crack dries level. But that is for info only your crack is less accessible.

You're asking great questions and I wanted to compliment you on how you are approaching this, good going.

_________________
Ann Arbor Guitars

Good to know. Thanks for this! I was pretty sure mineral oil wold work but assumed it would take a little danger to dry off so I didn’t want to use that first.

Thanks for this! I have plenty of Titebond original around.

Funny about the mineral oil, I was noticing that they only carried some “substitute” version of that, too, at least when I went looking for Naphtha. I guess California is making it tough on us for everything fun.

Luckily I have real mineral oil so I’ll use that. I was a little worried that it wasn’t as volatile as Naphtha so I’d have to let it dry longer before gluing but perhaps that’s incorrect. I also have plenty of ER-style rare earth magnets so that’s no problem. I’ve pinched some skin off with them before so maybe there’s a bit of flesh still hanging off them that I can reacquaint myself to

NO, NO, NO on the mineral oil. That would soak into the wood and would prevent glue from ever sticking. Really should just skip the whole naphtha step, as you already know that the wood moves around the crack which tells you that this is not just a finish crack.

I don't know where you are getting your information from, but naphtha, mineral oil and paint stripper are nothing alike. You should be careful about ever getting information from these sources again. Personally, I learned a long time ago that paint store personnel may know about their products, but that is about all.

Barry’s right - mineral oil is nothing like mineral spirits. Mineral spirits is what Hesh recommended.

Thanks

Here's what just resulted from a Google search to the question "is mineral spirits the same as mineral oil?"

"No. While the two are related, they are not the same thing and they are used for different purposes. For example, mineral oil is used to add a layer of protection to wood, while mineral spirits are used to take away a layer (thin paint, degrease, and clean)."

Mineral spirits being a "spirit" flashes off quickly and does not soak the wood in any way that stays soaked. It's interchangeable with naphtha for highlighting cracks while not damaging lacquer finishes. Works great.

_________________
Ann Arbor Guitars

Just be sure you see my comment was mineral spirits not mineral oil.

_________________
Ann Arbor Guitars

Thanks Josh much appreciated.

_________________
Ann Arbor Guitars

I got some naphtha, which works great. As someone with a chemistry background I’m rather disturbed by how little info they have on product labels for these household chemicals.

So with no joke intended... can you see a wet spot? I use an eye dropper to drop some of it on the damaged area on the outside and then mirrors and flash light to see if the wet spot... is migrating on the inside.

_________________
Ann Arbor Guitars

If you’re in Oz, then Shellite is your naphtha of choice. In Aotearoa it’s sometimes called Fuelite. Great stuff and one or the other readily available.


Sent from my iPad using Tapatalk

Yes, it is indeed called shellite here when packaged as a solvent.

Yes... seen and felt through the crack.

I followed your protocol with Titebond original and some rare earth magnets to clamp it.

Although I didn't have all the supplies I really wanted to do this right, I followed some internet suggestions on filling the finish crack with epoxy.

I used slow setting epoxy and the "razor blade with tape on the ends" to level it when it was dry and was very careful to NOT let the blade touch the binding.

I had 4 problems/issues that I think I know the source of: 1) Finish has some swirls where I did the work, which I think are from me not having all the grades of sandpaper I wanted and am short on cash so I didn't get all the right grades for a better transition. 2) Wood is discolored from the epoxy. Maybe that's just what happens with wood and epoxy and I'm stuck with that outcome? 3) There was a crack that the epoxy didn't penetrate near the ding so in hindsight I should have either tried to lift the ding with heat/moisture, or inspect it better and remove that bit of finish before filling. 4) There was a bubble in the epoxy so when I leveled it with the blade it left a bit of a hole behind. I think I should have knocked down the raised epoxy and then filled it again before proceeding. I didn't see it, though, until the end when I was polishing, and didn't want to get more epoxy for a tiny hole but if it was a nicer guitar I totally would have.

Epoxy is too soft to use as a finish and I would not have used it nor do we use epoxy for finish repairs. So I can't comment on how to make it work, I don't believe it to be suitable in the first place.

Ah the Internet....

Anyway what you have now is not bad this was not going to be pretty or invisible unless someone invested more time than is suitable for the instrument.

FYI CA without accelerator is used for some finish repairs on polyester finishes. Not sure what a Cordoba is. It can be built up and scraped down and it buffs out well.

_________________
Ann Arbor Guitars

The epoxy was my idea, so I can’t blame the internet! It’s what I had on hand and it’s a special epoxy formulated for super hard finishes in other industries (I had it at work). It’s probably not ideal, so I won’t disagree.

I’ll try the slow setting CA glue next time. I’m almost certain my finish is polyester as you guessed.

Thanks!

What's done is done on this repair, but a comment to consider on future repairs involving dents and crushed grain: Try to steam the dents so that the wood returns to as much of its original shape as possible before gluing and maybe filling. And coating bare wood with shellac might help control glue penetration and discoloration. But I am no expert at finish repairs.

_________________
Peter Havriluk

I’ll try to raise the dent next time. It was also so close to the binding I was a bit worried about doing it but I guess you don’t learn anything by not trying.