All wood will pickup moisture in the rain. The best you can do is seal it very well. Double the coats you put on the end grain. 3 or 4 coats on the face and edge and double that on the end grain. Mar-k did a finish study. They all will fail either by moisture or UV. Water based paint or just stain can be reapplied where as a clear finish must be stripped. If its just discolored you can strip it, bleach it stain it and reapply the finish.
