Looking from my upstairs window i can see almost my entire streets back gardens. Almost all of them have greyish wooden fencing panels, but i'm sure this asn' by choice.
what caused this so i can avoid it in the future?
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It is possible that some of your neighbors decided that a natural patina of weathered grey looks appropriate on their fence boards. The dulling of wood fibers occurs if the fence boards are left un-sealed or bare for several seasons. The greyish white tone that develops is mostly due to the bombardment of U.V rays from the sun. Without protection the wood fibers become dry and bleached. With the proper wood sealer and maintenance you can extend the life that a wood fence lasts for many years. If the fence is frequently subject to water (as when a sprinkler soaks it) the minerals in the water can tint the wood white when the water evaporates.
Exposed wood weathers - wood that is dry tends to weather brown, wood that gets wet weathers gray (you can get a delightful brown/gray transition on the sides of a barn where the upper part is kept dry by the eaves and the lower part gets wetted.)
If you have an issue with that natural process you will be painting, staining or varnishing your fence a lot/frequently to prevent it.
Seems to be the fashion these days to leave wood to weather. Once upon a time we couldn't give teak garden furniture away because of resistance to the yearly ritual of scraping, sanding and oiling. Nowadays folks just leave them to weather to a 'lovely' silver grey and everyone is happy; paying good money for neglected furniture...