Can lacquer checking show up months after temperature shock?

[Glennwillow]

I'm sorry about your surface finish cracks.

I know that there has been a lot of study of crack propagation in metals and I imagine that the mechanism is the same in a lacquer finish on a guitar. Once a crack has started, it's easier for that crack to grow.

I am guessing that the cracks originally started because of low relative humidity (RH) but once the cracks were started, the difference in humidity may be causing stress that is enlarging the original cracks. And it's certainly possible for new cracks to form.

The comments from EZYPIKINS were very interesting. It appears that finish may be vulnerable to cracks like that.

- Glenn

[rstaight]

I bought a new Cort in November in St. Petersburg Florida. Wasn't an expensive guitar but it spoke to me. Besides my wife liked the looks of it and said buy it.

It has a thick gloss finish. In the store it looked good. I get it home in Indiana where the weather is less humid and a good deal colder. I take it in to the shop about installing a strap pin on the side of the heal. The owner of the shop says, "You have a crack". We determined it was a finish crack from its location.

I was kind of bummed. But as I'm taking it out of the case to put the strap on and check it out, there was another crack. Next to controls for the pre-amp. Right then and there I decided it was not leaving the house until Spring.

About a week later I was putting on a set of strings, I didn't care for what was on it. The strings are off and as I am cleaning the guitar, I hear a pop. I look at the side and low and behold, I have a third finish crack.

I was considering having them repaired but now, I may just live with it.

[Wade Hampton]

After I asked “What’s VOC content?” Sinistral replied:

Originally Posted by sinistral

”VOC” stands for “volatile organic compounds.” VOCs are toxic chemicals often emitted as gas from solids or liquids. Some VOCs may have severe, short- and long-term adverse health effects when inhaled, which is why they are regulated. The VOCs are even more dangerous when the substance they are in, e.g., lacquer, is aerosolized.

Thanks for answering my question.


whm

[rampix]

I’m waiting to hear more experts chime in but I think JonWint’s explanation is likely the cause.

And EZPICKENS gave a great description of the lacquer process and associated issues.

It may just be my imagination but guitars with lacquer checking or cracking always sound more open to me. I’ve even thought of intentionally encouraging it but don’t have the guts. Today would have been a perfect day for a little natural checking operation. It was -2F this outside this morning and 72F in the house. I figured 20 minutes outside on a stand and then bring it back in would have done it…but again, no guts.

[jamf]

Originally Posted by JonWint

The guitar likely stabilized in the seller's lower RH conditions causing radial shrinkage of the top wood. That shrinkage induced compression in the finish. The compression would tend to increase the film thickness. If the finish can creep, the thickening may be permanent (plastic, not elastic).

Your RH conditions are higher than the seller's. As the top wood Equilibrium Moisture Content increased the radial shrinkage was reversed and the finish is now placed under tensile force. It can't stretch back and cracks. that's why the cracks are perpendicular to the radial direction.

See: https://www.wood-database.com/wood-a...nal-shrinkage/

This explanation makes sense. How frustrating that it may have fared better had I left it out in an unhumidified room!

[JonWint]

The fortunate thing is that the previous shrinkage didn't cause cracks in the wood.

Was the guitar bought new by the seller? Maybe the wood shrinkage occurred before the lacquer was totally cured. Softer lacquer may shrink with the wood instead of resisting and causing the wood to crack.

[jamf]

Originally Posted by JonWint

The fortunate thing is that the previous shrinkage didn't cause cracks in the wood.

Was the guitar bought new by the seller? Maybe the wood shrinkage occurred before the lacquer was totally cured. Softer lacquer may shrink with the wood instead of resisting and causing the wood to crack.

Yep, the previous owner bought it new. I would buy it again at the same price if it already had these cracks, so I'm not upset.

What do you reckon I should do with it now as far as humidification goes?