Popular Woodworking 2007-02 № 160, страница 7

Popular Woodworking 2007-02 № 160, страница 7

Letters

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Unwanted Lasting Impressions on Projects a Perplexing Problem

When using shellac to finish a proj ect, I notice that on tabletops I continue to get impression marks when something heavy is placed on the top - for example a lamp or a telephone, even when felt pads are placed at the points of contact on that item.

I've used both pre-mixed shellac, such as Zinsser at a 3-pound cut. I've also mixed my own shellac using freshly bought shellac flakes mixed with the correct alcohol. These were thinned and applied at about a 1- to 1V2- pound cut. Each coat was sanded after drying. The piece was then wiped using a tack cloth. When sanded there was no gumming on the sandpaper, just fine dust from the shellac, so I know that it cured OK before the next application. Anywhere from five to seven coats of shellac were applied depending on the item being finished. The last coats are even thinned a little more. After the final coat is applied I usually wait 24 hours before doing a final rub, either with steel wool or pumice and oil, and then the piece is waxed.

I've also noticed that the same problem will occur when I French polish a piece. In time, however, sometimes months and sometimes up to a year, the finish is OK.

Am I placing items too soon on the top, before the final finish really cures to accept a heavy item?

The only thing that occurred to me is that maybe there's a problem in my final finishing phases. When I rub out a piece, either using steel wool or pumice and oil, maybe I'm not waiting long enough for the finish to cure before applying wax to the item. And maybe by applying wax to the finish too soon, I'm in essence sort of sealing the finish and preventing air to get in to enable the finish to cure properly. It's still curing but it's taking a lot longer to do so. Any suggestions would be greatly appreciated.

Mike via the Internet

The thinner you use wont make any difference after it has had time to evaporate out of the finish.

The most likely cause of your problem is that the shellac you're using is too old. Shellac begins deteriorating when it is dissolved in alcohol. The deterioration is slow; it takes six months to a year before you will really notice any difference in how hard the finish gets. But the conditions in which the shellac is stored could accelerate this. Heat destroys the shellac more quickly.

It's also possible that you rubbed or waxed the surface too quickly. Stick your nose up against the dried shellac and take a whiff. If you can smell any of the alcohol, wait longer. If you cant, meaning all the alcohol has evaporated, go ahead and rub or wax.

You're not sealing in the solvent, however. If rubbing or waxing too quickly is the cause of your problem, it's because some of the oil or wax was able to penetrate into the shellac. Not very likely, but possible.

If the conditions are hot when you make the impressions, then the heat would be a factor. Shellac is an evaporative finish so it's not very resistant to heat. It loses some of its hardness continued on page 16