Things You'll Need
Bucket of warm, soapy water
Boiled linseed oil
Metal bucket or can
Sunlight, moisture and dry conditions can sap the moisture from wood window sills, leaving them dull, rough and damaged. Although a treatment of pure, unadulterated linseed oil dries too slowly and actually promotes mildew growth, a solution of boiled linseed oil and paint thinner applied to the surface will rejuvenate and protect the sills, and restore the beauty of the the wood trim. After several layers of this mixture are applied, the formerly dry and lifeless wood will begin to glow again.
Wash away all accumulated dust, dirt and grease on the window sill. Use a soft cloth and warm, slightly soapy water. Fold the cloth many times to prevent any splinters in the wood from spearing through the fabric into your fingers. Rinse the sill with clean water to remove the soap. Allow the wood to dry.
Countersink any nails that have nail-heads protruding above the surface of the sill by using a nail set and a hammer to pound them back into the wood. If any of the protruding nail heads are rusted, use a pair of pliers to pull those nails all the way out of the sill. Replace them with countersunk, galvanized nails.
Use a paint scraper and a putty knife to scrape away any loose paint on the surface of the window sill.
Sand the surface of the window sill to smooth away any roughness and remaining loose paint. This action should also remove any wood splinters from the surface.
Vacuum the window sill to remove paint specks and sawdust.
Mix equal parts boiled linseed oil and paint thinner in the metal bucket or can.
Paint a thin coat of the boiled linseed oil and paint thinner mixture over all the wood surfaces of the window sill. Allow it to dry for 24 hours. Cover the bucket of boiled linseed oil and paint thinner solution tightly to prevent it from drying out; store it away from any heat source or flame.
Check the window sill after 24 hours: If it is no longer tacky to the touch, add another coat of the boiled linseed and paint thinner solution. Allow the window sill to dry for 24 hours.
Add another coat of the boiled linseed and paint thinner solution after those 24 hours have passed. Upon the completion of this step, the window sill should be restored. However, let it dry for another 24-hour period.
Allow three days after your last coat before sanding the sills and adding an alkyd primer, if you wish to touch up the paint on the restored window sills.
"Boiled linseed oil" is a form in which linseed oil is sold. Don't boil it yourself.
Linseed oil is highly combustible. Never store linseed-oil soaked rags or paper towels. Wash them, saturate them with water or burn them, so that they don't spontaneously ignite in the trash bin.