Horizontal surfaces are always the first to weather as they are typically more exposed to the elements, standing water and moisture ingress. Vertical surfaces tend to be less exposed and water just runs off.
The key to avoiding these issues is regular maintenance. To rectify this situation, use Methylated Spirits to remove dirt and grease from the exposed surface. Once cleaned, the exposed wood should be treated with a mould and mildew treatment such as Barrettine Mould and Mildew Cleaner to kill off any mould spores embedded in the grain of the timber. Once dry, treat the area with a wood restorer such as Osmo Wood Reviver Gel (6609) or Barrettine Wood Reviver to restore the original colour of the timber and then re-apply 2 coats of Osmo UV Protection Oil Extra.
An additional layer of protection that can be added prior to oiling is one-coat of Osmo Wood Protector (4006). This product is a highly water resistant base-coat, preserver.
An alternative to the above would be to sand the horizontal surfaces to remove the grey surface and then re-oil once new, fresh wood is exposed. We would however recommend that the wood is still treated with a mould and mildew cleaner prior to oiling to be sure that all mould spores have been eliminated.
The maintenance period for these products is always difficult to estimate as it is literally down to how much wind, rain and sun the wood gets. For example, the windows on a cliff top, sea facing cottage will require more maintenance than a sheltered town house.