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Deteriorating door finish

Does anyone know what this might be, and how I might go about restoring it?
The house is 106 years old and it's an external door from the dining room, leading to the porte-cochere.

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Re: Deteriorating door finish

Perfect example of a well-alligatored finish. My pet theory is that a varnish was applied later over an original shellac, perhaps with a layer of wax or furniture oil/polish in between, so the subsequent over-coats have crawled.
You can scr ape off the worst of it (very carefully not to gouge or scr ape to wood) then with fine sandpaper get down just as far as the oldest coat of finish (shellac) and then re-coat with more new shellac. Zinsser Sealcoat is best. It may cause the old finish to craze at first, but let it dry, sand again with 220 or 320, and keep applying more new shellac and it will be all right.
I restore old furniture finished almost as bad as this, and use a wet-sanding technique and then pad the shellac on, but this is architectural, no reason to French Polish the woodwork.

Re: Deteriorating door finish

One thing for sure, what ever caused the crawling, it happened decades ago and probably involved natural resin varnishes, especially since it was an exterior door. You would never have shellac on the exterior of an exterior door.

A simple wiping with de-natured alcohol will determine if the finish coat is shellac. Alcohol will immediately start dissoving shellac. Even if it does dissolve the finish somewhat, the shear amount of the alligatoring would make for a gooey mess if you try to strip with alcohol. I would prefer one of the strippers designed for removing shellac. Alcohol flashes to the air very quickly, leaving a gummy mess behind.

If the finish coat is varnish, again a suitable varnish stripper would be appropriate.

As Sombreuil suggests, there are many possible causes of alligatoring/crawling, one of which is surface contamination. Other causes could be a later coat of varnish put on before the previous is thoroughly dry, too much drier being added to the varnish, large temperature change after application, benzine having been used to thin the varnish rather than turpentine, etc. Only the painter who applied the varnish knows what was done, and he is probably long dead!

As Sombreuil suggests, Seal Coat, a de-waxed shellac, can be used to seal the re-finished wood and can also be used as a finish coat, as was often done many decades ago, however, I would not use it on an exterior door side which is exposed to the elements. Seal Coat, when used as a sealer, is also compatible with almost any other type of finish coat, including modern urethane varnishes.

Re: Deteriorating door finish

Thanks guys. This is actually the interior side of an exterior door. I can't remember what the exterior finish looks like at the moment (I'm out of town for work). Except that someone at some point decided they wanted a screen door, but didn't want to pay for or go to the trouble of finding one that fit for this very tall and thick door, so they cut out a board the width of the door and the height of the difference between it and a standard door, painted it white and mounted it up top, then installed an aluminum screen door in the now smaller opening. The screen door is long gone but the fittings for it and the board remain. :)

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