Darkening Cherry in Localized Spots

Advice on using artificial light to darken small areas of a Cherry cabinet that have been kept in the dark while the rest of the piece changed color with age and exposure to light. September 3, 2010

I have a client with an existing library built in that we are going to be reworking to improve storage. The main problem is that all of the shelves are adjustable but have never been moved, so over the last 3 years the cherry veneer behind the shelves has not darkened. The client wants to add some lower doors and a small counter, and wants to move the shelves and not see the lighter color lines that are very obvious. Is there a fix for this?

We talked about refinishing the panels with a slightly darker stain/glaze and topcoats. The original finish is a nitrocellulose based lacquer. Obviously these panels can not be removed and taken back to my shop, so I need a way to finish them in place without a lot of sanding or stripping.

Forum Responses
(Finishing Forum)
From contributor L:
Just expose the cherry to light for a while and it will fix itself.

From contributor G:
The difference will fade after the light strips see some UV from the sun. You could set up some lamps. If you attempt to touch it out by staining the light stripes, they will be dark stripes in a short while when the cherry under the touchup darkens.

The problem is just the way cherry is. Short of sanding it all down below the level that the wood has naturally darkened to, nothing can be done except to either wait for the newly exposed wood to catch up naturally or to hit it with sun lamps.

From the original questioner:

Thanks. That is what I was thinking. I just wasn't sure how long it would take the light wood to catch up with the dark. There is a good deal of natural light in the room but I am tempted to try the UV lamp method too. What type of lamp should I use in this application?

From contributor G:
I am not sure. UV is a spectrum of wavelengths, but from a bulb buying point of view, there are two bands. One is for lighting up posters (long wave?) and the other is for germicidal cabinets (short wave?). The poster one is harmless, the germ killer one is dangerous to look at (eye damage). I do not know which is the one that darkens cherry. Grow bulbs and tanning bed bulbs also come to mind. If you go to the manufacturer's website you can get wavelength spectrum charts for most any bulb. You will be surprised at the amount of wattage you need to use to equal natural sunlight on a sunny day.

From contributor M:
Coatings manufacturers test their finishes for resistance to yellowing using a variety of different light sources. One common low intensity bulb simulates sunlight that has been filtered through a pane of glass and operates primarily at 351 nm wavelength and will give years of aging in days. Full spectrum (FS) lights (or grow lamps!) can be found at a hydroponic supply store and will work as well, but may take longer. FS bulbs run off a regular fluorescent ballast as well. I'm not sure about the 351nm bulbs.

Where you're going to have trouble is at the interface between the cherry that was exposed already (dark) and the covered wood (light). Any further exposure will slightly darken the dark areas as well. The difference will be much less than the darkening effect on the unexposed wood though. You could try masking the dark areas, but you run the risk of having a line where the two sections meet.

Also, any of these UV exposure methods will be very hazardous to the eyes. Make sure that you don't look directly at the lights and that you can easily turn them on and off so the wood can be inspected regularly.

Depending on the existing colour change (you say 3 years), this whole process could take anywhere from 4 hours to 100 hours of exposure.