On May 7, 9:10 am, dean wrote:
Is it dirt? Sun bleaching? Something else?

Do deck sealers work by stopping the rain water and dirt from
penetrating, or by coating with a UV filter?


Yes, and yes...

Much of the gray is oxidation and UV, but all of the effects mentioned
plus molds, etc., are at play.

Sealers do a little against all of the above -- how much of what they
do, specifically, depends on the product and the situation. What the
wood is, the climate, etc., all are variables...