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My painted board has 2 coats of shellac & has dried for 2 days. But when I place a piece of décor on it, it leaves a mark.

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    Hi, Mabel, and welcome. I guess you probably mean to ask what you can do about the problem you describe, but it'd be best if you'd edit your post to state the actual question. You'll get better answer, too, if you include as much information as you can: a photo of the piece and the mark, the kind of paint, where you got the shellac and how long ago, how you applied it, how long you waited between coats, whether you thinned it with anything, and if so, what. – Caleb Jul 22 '18 at 4:01
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Shellac applied in thin coats normally dries very quickly, but it won't harden completely if the shellac is too old. Check the can -- there may be an expiration date that'll help you judge how old it is. If the shellac has been stored in a warm place, like a hot garage, that can shorten its useful lifetime.

You can check the condition of your shellac by spreading a bit on a piece of glass. The shellac should harden quickly; if it doesn't, and instead remains tacky or gummy, then the shellac is too old.

I'd let the piece sit for a while longer if you can -- maybe you'll get lucky and the finish will fully harden with more time. But if it seems like it's never going to harden, then you can remove the shellac with alcohol.

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    What is the mechanism of deterioration? I think shellac was just an fairly inert solid dissolved in alcohol; when the alcohol evaporates, the shellac is left behind as a film. – Martin Bonner Jul 23 '18 at 9:26
  • @MartinBonner That'd make an excellent question in its own right, as I'm not sure the answer is simple enough to fit here (or to be known by me). Bob Flexner wrote that it's a reaction between the alcohol and resin, and The ageing & stabilisation of shellac varnish resin... seems to agree. – Caleb Jul 23 '18 at 14:30
  • @MartinBonner As wel, Degradation of lac with storage and a simple method to check the same suggests some ways that manufacturers can improve shelf life, which may help answer your question. – Caleb Jul 23 '18 at 14:32
  • So in short, it seems that the resin isn't quite as inert as one might assume it to be. – Caleb Jul 23 '18 at 14:34
  • There's a lot of user-base info on the issue of old shellac not hardening properly which I wouldn't discount normally because anecdotal information forms a body of evidence regardless of it not being regarded as formal evidence. However, this is an area where people can't or won't wait if there's a drying problem — they need to finish the project, often now — and this leads to the common belief that it won't harden when in fact it's more common that it will just take longer. Sometimes much, much longer, but still that isn't won't harden as it's usually stated. – Graphus Jul 24 '18 at 12:23

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