From that FAQ:
"Browning of White Sidewall Letters
BFGoodrich® tire designs include protective agents to retard the natural degradation of the rubber over
time, which can occur when exposed to the sun and to ozone present in the air.
These protective agents are designed to slowly migrate to the surface over time in order to provide
continual protection over the intended lifespan of the tire.
Under normal circumstances the protectants are not noticeable on black rubber or on white rubber.
However, under certain circumstances, these protective agents may become noticeable over time as
yellowing or browning due to interaction with ozone. The contrast may be most notable on white rubber.
Discoloration of the white rubber is not a warrantable condition.
At the same time, exposure to sunlight has a beneficial effect on the appearance of the white rubber.
The micro-thin outer surface of the white rubber is designed to photo-oxidize when exposed to sunlight.
Rain, car washing and even the flexing of a rolling tire will progressively remove these oxidized particles
from the white rubber. As particles of the micro-surface layer are removed, new white rubber is revealed.
The white rubber of tires on vehicles that are garaged or protected from regular sunlight exposure are
not allowed to oxidize or to refresh their surface appearance as designed.
As a result, the migration of protectants can become visible over time on tires not exposed to sunlight.
Tire sidewalls or letters which have become dull due to dirt or similar buildup can be cleaned with mild
soap and water. Should discoloration from migration of protective agents occur, regular exposure to
sunlight will refresh the white rubber.
If regular exposure to sunlight isn’t possible or practical, the following hand cleaning methods could be
employed:
Cleaning Option 1: Use common steel wool soap pads (such as Brillo or S.O.S. brands) with water on
the white rubber to gently remove surface discoloration until the white color is restored.
Be sure to rinse thoroughly to remove any remaining cleaning particles.
Cleaning Option 2: Use wet sanding paper (400 grit) with water and mild soap on the white rubber
to gently remove surface discoloration until the white color is restored. Be sure to rinse thoroughly
to remove any remaining cleaning particles.
BFGoodrich® Tires does not recommend any powered cleaning methods, either by mechanical,
hydraulic, pneumatic, etc. For additional information, please contact your local BFGoodrich® sales
representative or contact BFGoodrich® using the website at
www.bfgoodrichtires.com"