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Thread: Is there a treatment or cure for checking finishes

  1. #1

    Default Is there a treatment or cure for checking finishes

    The finish on my vintage instrument is checking, under the strings. My pick does not go that deep when I play so it is not caused by pick strikes. The instrument was worked on and refinished before I acquired it, probably about 25 years ago and the finish of the top has been in remarkably good condition by I have just noticed hairline flaws in the finish that I am sure were not there a few months ago. I don't know whether the current finish is French Polish, lacquer or varnish. The room where the instrument is kept has a relative humidity ranging from a low of 30% to a high of 50%. It averages in the 40+ range on humid days and in the low 30s on dry days. When I play it, I run a humidifier a few feet away from where I play. I don't think that a very low relative humidity is the cause of the checking but I could be wrong. Is there something that one can put on the finish to protect it (and that will not kill the sound)? A wax? A preservative of some kind? Is there a special cleaning method that will prevent checking.

  2. #2

    Default Re: Is there a treatment or cure for checking finishes

    Finish cracks on lacquer instruments are virtually impossible to prevent in the long run. It's usually a relatively quick change in temperature that is the culprit, not so much humidity. There's not much can be done that's non-invasive, if you will, and that wouldn't negatively effect the value.

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  4. #3
    Likes quaint instruments poul hansen's Avatar
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    Default Re: Is there a treatment or cure for checking finishes

    In the guitar world there are som peoples that like crazing so much, they freeze their instruments and then heat them, to produce it. It usually happens with Nitrocellulose Lacquer used a lot in th 50s 60s. I've never seen it on French polish/shellac. But if it is(can be testet with a spot of alcohol/spirit on an invisible place. shellac is soluble with alcohol) then it's easy to repair with one stroke of alcohol or shellac.

    Sell it to craze lovers and buy a new one.
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    Default Re: Is there a treatment or cure for checking finishes

    If you're not already doing so, maybe try a case humidifier, at least for periods when you know the RH is going to be in the lower 30s. I have a whole house humidifier I run from November to about this time of year to keep RH 50% +/- in our drafty 1938 built house, and I also run dehumidifiers in the late spring to October or so (I'm near Charlotte, NC). It may (or may not) make a difference, as I agree this tends to be more of a rapid temp change or just aging finish issue than a humidity issue. I'm not aware of any polishes/waxes/etc that would slow this process down.
    Chuck

  6. #5

    Default Re: Is there a treatment or cure for checking finishes

    Quote Originally Posted by poul hansen View Post
    In the guitar world there are som peoples that like crazing so much, they freeze their instruments and then heat them, to produce it. It usually happens with Nitrocellulose Lacquer used a lot in th 50s 60s. I've never seen it on French polish/shellac. But if it is(can be testet with a spot of alcohol/spirit on an invisible place. shellac is soluble with alcohol) then it's easy to repair with one stroke of alcohol or shellac.

    Sell it to craze lovers and buy a new one.
    It's over 100 years old and they stopped making them in the 20s. There AREN'T any new ones. I will look for an invisible space, or live with it. Thanks to all for the advice.

  7. #6

    Default Re: Is there a treatment or cure for checking finishes

    Will the crazing affect the tone of the instrument?

  8. #7
    The Amateur Mandolinist Mark Gunter's Avatar
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    Default Re: Is there a treatment or cure for checking finishes

    IMHO no, checking will not affect the tone of the instrument in any appreciable way.

    There are ways to repair lacquer finishes. The facts that nitrocellulose lacquer is reversible, and that it dries quickly, are properties that made it so popular in the last century. Lacquer is reversible in that it can revert back to a liquid state with the application of lacquer thinner.

    There is a very slow-drying lacquer solvent called retarder. Finish professionals use bulk retarder to add to lacquer in order to slow the drying process, which is useful for getting better flow-out or for preventing blush (which occurs when moisture is trapped in or beneath the lacquer). But there is another use for retarder: Proffessionals spray retarder on finished lacquer surfaces in order to melt down an area and cause the finished area to liquefy. This can cure small defects and also remove blushes. Finishing suppliers may package retarders in aerosol cans for this purpose. My main supplier, Mohawk Finishing Products, carries several such aerosols under the names NoBlush, Blush Retarder, Super NoBlush, etc.

    So, short of refinishing, there are a couple of approaches that work to repair some checking. One would involve treating the area with retarder. Another would be treating the area with “amalgamator”, similar to padding over shellac with alcohol. The amalgamator material is a proprietary mixture of alcohol and other solvents that can soften lacquer and allow the surface to flow a bit when padding it.

    I wanted to give this overview to argue that a skilled finisher or luthier may be able to help, though they wouldn’t give you a guarantee of success. The products would be available to DIY folks, but things can go wrong. There is no way to know whether silicone has been introduced to the finish in the past, for one thing. In 100 years any number of contaminants may have been introduced which may react unpredictably when lacquer is re-liquefied. You could make things worse, in other words, and without skill and experience correcting things that may go wrong, you’d be really unhappy if you make things worse.

    The checking should not affect the tone. Play it and be happy.
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