UV-resistant, non-yellowing finish

      Should claims of UV-resistance in topcoats be trusted for the long haul? September 6, 2000

One of my customers wants to switch from laminate to white oak backs in store fixtures which are often exposed to direct sunlight. He is talking to a designer who assures him that yellowing is no longer a problem due to new finishing technology. I told the customer this is not entirely correct. Do the new UV-resistant finishes really resist yellowing that well?

I have seen a solvent-precatalyzed lacquer sample that was exposed to direct sunlight for several weeks. The wood did not yellow or change color one bit. But would I take on a project and assert to my customer that the wood will not change color and the finish will not yellow or amber? Not me. If there would be color change then I would be in trouble, and who needs the headache?

That said, Guardsman/Lilly adds a UV block to their coatings that is said to be quite special. I know of a couple of shops using their coatings and what I can tell you (having seen it with my own eyes) is that these coatings work for them just as I have described: no color change at all.

There are certain waterborne coatings that do not yellow. But that does not mean that their applications stop the substrate from exhibiting a color change. Will the march of time and sun exposure allow a patina to develop with woods coated with these waterbornes? That's what I would be asking each finish supplier, and I would scrutinize their every claim.

Look for an acrylic finish. Any finish that has nitrocellulose (NC) in its makeup will yellow. When you use a finish that has a UV absorber, you are only slowing down the yellowing. At some point that block will get used up and the product will start to yellow. Acrylic finishes hold their clear color better than any other coatings.

Look at the chemistry of the product. This will tell you more than anything else.
Bob Niemeyer, forum moderator

I agree; I have used Sherwin-Williams acrylic lacquer with their vinyl sealer for seven years, and never had a yellowing problem. It's especially good for paint-grade white, which is very difficult to prevent from yellowing. I use a white vinyl sealer tinted for color, then their acrylic topcoat.

I don't know anything about conversion varnishes and how they would hold up as far as yellowing.

The UV block in the Guardsman/Lilly coatings is said to be non-sacrificial (i.e., does not give up its UV-blocking qualities). How much puffing there is to that claim I cannot say. But they subjected their samples to tests mimicking 100 years of exposure to sunlight. The results were enough to impress them.

This is correct; I verified this with Lilly and have tested this personally. Although my tests were nowhere near the 100-year mark, other non-NC lacquers I compared did not fare as well. While the coating held up pretty well, the underlying wood did not; fading was quite evident. I also agree that some acrylics will hold up "well enough" for practical purposes, but I would not extend this statement to acrylic-modified NC lacquers due to the NC - as was noted.

We have had great success with Therma-Tru water-based clear coats, as far as non-yellowing goes.

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