Water and wood are best separated by a well-applied layer of quality paint if a window sill (or any other wood component exposed to the elements) is expected to last. The previous owner of our house did not kept up with painting, allowing several to deteriorate beyond the point of patching.  Today I spent the better part of the day replacing two of four. Weather permitting I will tackle the remaining pair tomorrow. Not particularly the kind of woodworking I enjoy but a long overdue repair that simply needed to be done.