If your window sill looks like the picture on the left, you have 3 choices.
#1-You can paint it, but that is a temporary solution. Besides, the window sill wood is still exposed to the effects of weather.
#2- You can hire a contractor to wrap the window sill wood in aluminum, which will protect the wood from the elements. However, the labor costs can be high, white aluminum doesn't match the color of white vinyl, and the installation requires using nails, so the nail heads are quite unsightly. And if the aluminum gets scratched, it exposes raw metal which needs to be painted to prevent rust.
#3- The third option is to cover your window sill with a vinyl window sill repair cover. The Vinyl window sill cover addresses every concern. It eliminates the need to ever paint again. It protects your wood from the weather. It can be installed using a sealant like silicone, so there are no nails or screws involved. It matches most white vinyl window frames. Finally, any homeowner with basic tools can do the job themselves. The dimensions of the window sill cover are 4 1/2" on top, 1 3/4" on the face, and 7/8" underneath. To purchase the sill cover, use the drop down menu on the 'Shop" tab and select "Sill Covers".