Do you have a window or windows that seem to peel every year. The culprit is almost always moisture. Moisture from rain and snow eventually penetrates the paint film from cracks . When it evaporates, the pressure from under the film causes peeling. To stop this, sand the window sill and then caulk all angles and crevices. This will help eliminate any place moisture tries to penetrate the wood. Then with the appropriate primer, paint the window sill and let it dry. The last step is to paint over it with the paint of your choice. Fort Collins house painter can help you with this problem.