Remove all peeling paint using a paint scraper. Eliminate peeling paint from corners and grooves with a wire brush.
Sand the broken edges of paint still clinging to the window frame, sill and trim work with 120-grit sandpaper. Sand until all areas are smooth to the touch.
Brush away all paint particles with a dust brush and clean the grooves in the window sill with a vacuum.
Soak a paintbrush's bristles with exterior primer paint. Prime the sill, frame and trim work with the brush. Wait 24 hours for the primer to dry.
Caulk the joints in the trim work, window frame and sill with a caulking gun loaded with a tube of acrylic siliconized caulk. Cut the tip of the tube of caulk off with a carpet knife. Smooth out the caulk with your finger and then allow it to dry for 24 hours.
Paint the sill, the window frame and the trim work with exterior finish paint. Allow the paint to dry for 24 hours and then apply a second coat.