Combine 1/4 cup trisodium phosphate and 1 gallon water in a bucket. Dip a sponge in the mixture and wring it out. Wipe down the old window frame and glass. Pat it dry with old clean towels.
Attach a piece of 100-grit sandpaper to the bottom of an electric sander. Run the sander across the surface area of the window frame. Repeat with 220-grit sandpaper and then
360-grit sandpaper. Wipe down the window frame with a tack cloth to eliminate the dust from sanding.
Measure the width of your window frame. Write it down. Visit a lumberyard and purchase two boards that are as long as the width of your window frame and 6 to 8 inches wide.
Dip a 3-inch nylon brush into the paint or stain of your choice. Cover the window frame and boards with paint or stain. If using stain, allow it to soak for 20 minutes before wiping it off. Repeat twice. If using paint, apply one coat and allow it to dry overnight.
Drill in one side of each L-bracket to one end of the bottom of each board. Position the boards horizontally across the window frame. Drill the other end of each L-bracket to the respective vertical sides of the frame.