Spread plastic tarps over the floor under the windowsill to prevent the paint remover from damaging it. Apply painter's tape to adjoining wood trim and the window. Raise the window, if possible, to help vent the fumes outdoors.
Put on safety glasses and rubber gloves to protect yourself from the harsh chemicals in the stripper.
Open a can of gel-based paint stripper and insert a paint brush into it. Apply the stripper to the wooden windowsill liberally until all of the stained portions are covered in a thin coat.
Read the instructions on the paint stripper can to determine how long the gel must sit and wait the appropriate amount of time. Some strippers can take up to 20 or 30 minutes to work.
Place a plastic putty knife on the windowsill at a 45-degree angle and push it forward to scrape off the gel and the wood stain. Shake the removed material into a bucket to collect it and repeat the process to remove a much of the gel as possible with the putty knife.
Scrub any tight crevices on the windowsill with a fine-grade steel wool pad to remove any remaining traces of the wood stain.
Fill a clean bucket with 1 gallon of water and 1 tbsp. of a strong detergent. Insert a scrub brush or sponge into the mixture and scrub the windowsill to remove all traces of the paint stripper.
Wipe the windowsill dry with a towel. Then soak a rag in paint thinner and wipe all portions of the windowsill with it.