Put on the work gloves. Measure one cup of paint thinner in a measuring cup. Place the cup inside a bucket. Place sheets of newspaper up against the bottom of the wall where the window pane is located, to catch any drippings from the cloth that you are about to use on the window.
Wad up a soft cloth and place it into the mixture inside the cup so it is totally covered. Leave the cloth in the mixture until it is soaked. Remove the cloth from inside the cup.
Unwad the cloth and place against the tung oil stain on the pane of the window. Hold the cloth against the stain for at least 10 minutes.
Wipe the stained area of the window with the cloth, using a firm, circular motion. Continue to do this until you can no longer see the stain. Place the cloth back in the bucket, but not in the measuring cup that is inside.
Wash the area of the window where the stain was located with a soft cloth moistened with water or glass cleaner, using the same slow, circular motions as before, for three or four minutes.
Return the contents of the cup to the paint thinner container. Dispose of the cloths in the trash, after allowing the one that had paint thinner on it to dry out. Wad up the newspapers and dispose of them in the trash.
Wash out the bucket and cup with soap and water and let dry.