I use Klean Strip paint stripper. I brush it on and use a putty knife to scrape off the old paint. It works on auto paint, but usually only one layer at a time. When down to the final layer, what won't scrape off is softened quite a bit. I use a wire brush on a drill to remove what's left. It is work and time consuming. The upper portion of my door had 3 layers of paint and one primer coat. It took 2 hours for me to do that section alone.
