How to Make a Window Sill From MDF

Measure the width and the depth of the window base with a measuring tape. The width is the length of the bottom of the window and the depth is the thickness of the window frame. Add 1 inch to the depth and 2 inches to the width. This provides the shelf for the window sill.

Purchase the lumber as close to the final measurements as possible or cut it to size using a saw. Measure a 1-inch length inward from both short sides of the piece. Mark the measurement on the board. Mark the depth of the window from one of the long sides on the board. Connect the two sets of marks to form a cutout of the corners along the same long side of the board.

Cut the lumber along the corner cut-out lines with a saw. You should have a T shaped board. Use a fine-toothed saw for smaller sills or a tenon saw for larger window sills.

Fit the sill into the window opening. Sand the interior edges with medium-grit sandpaper if the sill sticks or fits too tightly. Mark the positions where screws will go to attach the sill to the frame every 3 inches across the sill length.

Remove the sill. Drill pilot holes into the sill at the marked locations. Drill straight shank drywall screws into the pilot holes, continuing down into the window frame. Counter-sink the screws so the heads are flush with or just below the board face. Use of these screws will reduce the chance of the MDF board splitting.

Cover the screw heads with wood putty using a small putty knife. Allow the putty to dry. Sand the surface smooth.

Apply a coat of oil-based primer to the surface of the window sill. Avoid water-based primer because MDF board absorbs moisture. Oil-based primer creates a protective coating resistant to moisture. Apply two coats of weatherproof paint to cover the surface. This allows the sill to resist humidity inside and weather outside.