Help from my Friends

[rich_briere]

Hi Guys,

My friend and soon-to-be-member just brought in his McGuinn. Last time he opened the case it was fine. This time the guitar has a bunch of finish cracks that have appeared. Any clue as to what could have happened and, most importantly, who's the best "Mr. Fix It" for a situation like this??

Bass-ically Yours,
Rich

[lars]

Finish cracks are usually caused by quick temperature changes. From cold to warm. That's the way some makers "relic" their guitars: put it in the freezer over night, then take out and the finish cracks. Due to the wood expanding quicker than the finish.

What to do to fix it is another story. Best told by the excellent refinishers here.

[beatlefreak]

I'm not sure there's anything that can be done to fix it, short of a refinish. I think the only thing you can do is take steps to keep it from getting worse.

[tennis_nick]

Not much of a temperature issue as it is a humidity issue.

[jingle_jangle]

Nick, not to be adversarial, but sudden temperature changes are the culprit in the majority of cases. although extremes in humidity can be at fault in some.

It's going from a very cold environment to a suddenly warm extreme that causes the wood to expand faster than the varnish skin can cope. Result: checking.

[rich_briere]

I knew I could count on you guys. :^>) Mark is certain that there were no temp changes but that the humidity thing is a possibility. His home was totalled in the floods that we had and about 40 guitars went through it.......not underwater, but one floor above the river roaring through his living room. The McGuinn appears to be the only one that wigged out so I wondered if there might be something about the Ric finish that was more likely to be affected.

Bass-ically Yours,
Rich

[tennis_nick]

Paul, it seems we're trying to prove the same point, as temperature and humidity are very closely related, unless it's in a lab somewhere you know. But as I recall, John Hall had stated on the Rickenbacker Forum that The finishes themselves are capable of withstanding temperatures much higher and lower than any guitar will ever really be exposed too, but that the shifts in humidity directly related to the rise and fall of temperature are what cause the finish cracks, as seen in most Gibsons 35+ years old.

[jingle_jangle]

Then why do we call it "thermal shock", Nick?

Humidity does not make wood expand and contract rapidly enough to outstrip the ability of the finish to compensate.

There's no question that conversion varnish (generic name) will withstand some pretty wacky temperature extremes. Rick does put samples of any new varnish, sprayed onto wood substrates, through heating and freezing cycles, to ascertain its ability to cope.

If it is humidity that causes finish checking of conversion varnish, I'm going to stop wet-sanding my refins!

An old Gibson or other guitar that is finished in a thin shell nitro or French polish, is much more susceptible to damp than is a modern Rick with CV.

[dale_fortune]

Here's my 40 years of experience...Humidity (moisture in the air) is absorbed into the wood and will cause a swelling, this in turn can cause the finish to crack..Ever notice that automobile metal work does not expand in the rain, no finish cracks there. Low humidity will cause the grain in unfinished wood to open up and split(crack). I have done many vintage(aged) finishes by putting the instrument into a freezer for several hours(not overnight) just long enough to freeze the moisture content in the wood, take it out and place it in a warm room, the finish will check almost instantly as the wood begins to expand while the finish stays the same. Age has a lot to do with this process also, since the solvents tend to release over the years making the finish more brittle. As for C.V. or lacquer, if water gets into any open grain or screw holes that are not sealed or treated, the wood will expand and checking, cracking will take place.. the best you can do is live with the checking in your RM or have it refinished.once the checking has taken place you can not turn it back.....

[byu]

I recall hearing that checking along the grain of the wood is most likely due to changes in humidity. Horizontal checking is most likely due to exposure to rapid temperature change.

I've seen a lot of McGuinns (including mine) that have checked along the grain.

[dale_fortune]

I have a Jetglo RM 12 string in my shop that has severe checking in all directions, I suspect it was subjected to extreme temp. changes. When the wood/moisture content gets very cold, it shrinks, then when it warms up quickly it expands causing the finish to check. The finish itself is very stable, but the wood isn't. It will expand and contract with the humidity and atmospheric pressure.