- Q.Most species of trim that
we install become darker as they age. I’ve
read that this darkening is caused by exposure to the
ultraviolet (UV) rays in sunlight. Occasionally, a
customer will complain about this darkening. Is there a
clear finish available that will block this UV
action?
A.Tom Brown
responds: Exposure to sunlight will cause most
species of wood to darken, but the darkening is not
caused exclusively by UV radiation. Color changes
occur when the pigments within the wood oxidize.
This oxidation is caused not only by the "free
radicals" generated by UV radiation, but also by
the presence of oxygen. The UV absorbers found in
many finishes inhibit or partially block UV
radiation from reaching the wood, but since no
finish is totally impermeable to oxygen, the color
change will still occur, though more
gradually.
To postpone the inevitable, I recommend using a
film-forming finish, which blocks oxygen
penetration much more effectively than a
penetrating finish. Make sure the finish contains a
UV "absorber," not a UV "inhibitor." UV inhibitors
are used to protect the finish film from UV
degradation, and do not protect the wood pigment
from UV exposure as effectively as UV
absorbers.
Tom Brown is a wood finishing
consultant in Ft. Myers, Fla.