Aircraft paint stripper. Comes in an aerosol can at your local auto parts store. Works well. Use heavy duty rubber gloves and a wire brush and a plastic scraper. Spray it on in a well ventilated room, better if you're outdoors. Leave it on for about 15 minutes. scrape off the residue. Repeat. Don't let the stuff sit for long as it'll just dry up and make it harder to remove the paint. I DIYed my fixie frame and spraypainted it a nice Chevy Orange using Duplicolour Enamel engine paint. Came out nice but I think I may get her sandblasted and powdercoated over the winter along with my singlespeed mountain bike.