CB is right, use a stripper if the frame is painted. Make sure it's a "water base" stripper and you use only a brass bristled brush for those stubborn spots. You'll thank yourself later. If you need to remove oxidization, rust from metal parts and stains then I would recommend an aluminum cleaner made by Mary Kate products. I've used it for years in the marine industry. Just brush it on, let sit for about 10 to 15 minutes, then rinse with water. It can be had at just about any marine supplier round the world.
BTW, Mary Kate also makes a product called ON / OFF. Don't use on/off unless you want to watch the aluminum disappear before your eyes. Only use the Mary Kate, Aluminum Cleaner.
Remember: All things will yield with the proper doses of chemical abuse.