How important is shellac age for a very thin cut?

slateberry51November 16, 2009

I'm slowly working my way through an extensive panel door refurb in my house--so slowly that I'm worried I need to throw out my shellac and get more.

The doors are poplar, and to get a good finish that matches the original woodwork in the house (which is also poplar), I go through a multistep staining process that layers on the color for an even, rich finish. The first step is a washcoat. For that I use a thin cut of dewaxed shellac in denatured alcohol. A few months ago I did the master bedroom door with a 2 year old mix. It came out fine. Now it's a 2 1/2 year old mix. I'm thinking I'm really pushing it on the esterification. But, maybe for a washcoat application, the esterification isn't an issue--perhaps it only matters for a top finish? Perhaps I should thin the mix with less denatured alcohol, essentially making it a stronger cut, to compensate for the esterification?

On the one hand, I can see the virtue of buying new and starting over. Otoh, I hate to spend the $30 to buy a new gallon and throw away half a gallon of the old stuff, if it's still functioning for the particular needs of what I'm doing with it.

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Why not put a few coats on a piece of scrap lumber and find out whether it dries properly or not? If it's shot, it's shot -- no point in trying to use it up. If you need to buy more, consider buying a quart can instead of a gallon. Or, order some dry flakes and mix your own as needed.

    Bookmark   November 16, 2009 at 12:02PM
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Hey Jon, thanks for the tip. I did not know that the esterification interfered with proper drying. That gem of info. set me on a search that yielded a longer discussion of shellac esterification, which happened to mention that in 2002 zinsser obtained a patent on a shellac that has a longer shelf life, even when mixed in a thin cut. Turns out this zinsser seal coat is what I was using! No wonder it was still good. Talk about dumb luck! So the technical discussion gives it a 5 year life, zinsser guarantees for three years, so sounds like I'm safe for at least a couple more months.

What I will do is discard the batch I mixed in a small can with more denatured alcohol, and mix up a new batch from the big can. Apparently the thinner the cut, the faster the degradation, so my custom batch would be the first to go.

Now I don't feel like I'm stumbling around in the dark anymore, but I think next time I'll buy the flakes like you suggested so I don't have to worry about this so much. I think the flakes might degrade even more slowly than I progress on my projects :-)

Here is a link that might be useful: how shellac goes bad

    Bookmark   November 16, 2009 at 1:19PM
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If you purchase flakes you can dissolve them on denatured alcohol as needed.

The flakes have a very good shelf life.

    Bookmark   November 16, 2009 at 4:03PM
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