Not all deck sealers or water repellents are created equal. Film forming deck finishes contain clear resins that soak into the wood but they also lay at the surface. A film on any horizontal wood deck or railing is not desired. Penetrating water repellents are a much better choice. They soak deeply into the wood and stop water penetration.
Intense ultraviolet (UV) rays in sunlight blast away at horizontal deck surfaces during the mid part of the day. These same rays glance off vertical railing pickets. This is why your flat surfaces always look worse than the railings. Add to this the fact that rain and snow will puddle and stand on the horizontal decking and flat hand rail tops. The UV rays and water vigorously attack the film finish. The net result is that the weakened film finish peels off rapidly and readily.