Use a stripper. Citristrip is non-toxic and works well. You need to be careful when scraping as it can gouge wood. After applying the stripper and letting it sit for a half hour, gently scrape with a plastic scraper. Reapply stripper as needed and repeat process. Since mullions have small areas that trap paint, you will have to be resourceful about what you use to get if out of crevices. I have used everything from rug remnants to a threaded needle.

Once you are at the point where there is just a dab of paint here and there, rub it out with denatured alcohol.