finish checks?

[Kevin Briggs]

Hi, everyone.

I just noticed some wierd finish checking on the back of my mandooin. If the light isvshining the right way, I can see a bunch of checks around the edges of the back and going into the middle. They just look like short lines going in different directions. I tried buffing them out breathing on them and rubbingbthem using my cotton shirt, but they appear to be below the surface.

I almost never take my mandolin out of the house, and I control the temperature and humidity. It is an F style with nitro finish.

Thoughts on why it uappened and how to fix it?

[tburcham]

Kevin

Maybe Big Joe or one of the Cafe luthiers will chime in to give you a scientific answer for the "checking" on nitrocellulose finishes.

[Paul Hostetter]

Finish checks like what I think you have are often called cold cracking. They result from a sudden move from cold to warm, where the wood expands while the relatively hard and brittle nitro lacquer cracks, as it can't change with the wood quick enough. An instrument only has to experience this temperature shock situation once.

Sometimes they just happen, a factor of the lacquer's original chemistry, or thickness, whatever.

There's also the possibility that the surface lacquer is reacting to the sealer coat beneath it. This is more common in more modern CP-type finishes than nitro lacquer. I'm only throwing out ideas because I can't see your instrument.

There was a thread about this issue, why not look there (http://www.mandolincafe.com/forum/showthread.php?57058-Cracked-Lacquer) for some images? Here's one from Frank Ford's piece on this guitar.

http://www.frets.com/fretspages/Musician/GenMaint/Heat/73D35Heat/73D35HeatViews/73d35heat07.jpg

I wouldn't worry too much about it. There are some possible measures for dealing with it, but it's also part of the beauty of older nitro finishes.

[big smiley guy]

There are different reasons, some of which have already been stated. Other than age and cold checking other things that contribute are stresses (either sudden or over time) exerted on the instrument. Thicker finishes will tend to check more as well.

[Big Joe]

Finish checking is a natural part of nitro cellulose lacquer and can be caused by many reasons. The finish will cure to a point in a very short time, but will continue to shrink and cure for a long time. There will be a point where the shrinkage will cause the finish to check. It does not harm the instrument, or the finish, and is not anything to be concerned about. The shrinkage can come from exposure to rapid temperature changes or from age as it continues to thin but it will happen sooner or later. If it bothers you it can be fixed, but not always without more work than you may wish to engage. Worst case is that it has to be refinished. If you use nitro again, it will develop checking again sooner or later. Even poly finishes and varnish can check. Some check easier than others, but most will at some point.

I don't know what the reason for your to check may be, but be assured it is a natural occurance with nitro lacquer and does not harm your instrument. In the vintage market, one of the first things the experts or collectors look at is to see that the finish does have checking. If it does not it can get a bit interesting. They begin to worry the finish is not original, which does affect the value of the instrument. For many, it is a very desireable effect. Anyway, this is a brief statement on what happens. Hope it helps.

[bluegrassdan]

So what is the the best finish to avoid checking? Or the best period?

[Paul Hostetter]

Shellac (French polish) doesn't check. Neither does oil varnish. Ideally, catalyzed polyester finishes don't either, but I sure have had a streak of checked finishes like that lately. There's no "best" - they all have pluses and minuses and various reasons to recommend themselves. Less is more when it comes to finishes, in my opinion. Ouds and some older Italian mandolins have no finish at all.

[big smiley guy]

Too much hardener or too high humidity can cause catalyzed finishes to check. Rapid temp changes can as well but it tends to check differently than lacquer - usually long cracks.

[Big Joe]

Even oil varnish can check and so can poly. It has a different appearance, but they will. The varnish can often be taken care of with a good job of french polishing the area. Nitro is most known for the checking, but I have yet to see a finish that cannot check....even if it is a rare occurrence. The best finish is the one that meets your requirements. Each finish affects tone so one has to decide which finish will satisfy them tonally as well as aesthetically. The only other factor is expense. Some are more expensive to apply and care for. I prefer the tonal advantages some finishes deliver over the more exciting shine others may have. Just a personal decision.

[bluegrassdan]

Someone recommend Deft lacquer. Dont know if it is nitro. Any thoughts? What is good just at my local paint store?