Same as removing paint from wood. I would use a product like CitriStrip. Remove the metal parts, paint on the stripper or put the parts in a metal pan and pour it over, let it sit for half an hour, wearing chemical resistant rubber gloves and eye protection, scrape off the old paint with a putty knife. If any of the paint residue does not come off, get in the cracks and crevases with a brass brush or dental tools (both of these are available at large specialty tool stores). Any Home Center has paint strippers but be sure to use skin and eye protection when using them and follow the manufacturers directions. Depending on the base metal of the knobs and hinges, you may wish to have them polished after stripping. You can use a rotary (Dremel) tool for this or a polishing wheel or buffer. If you don't have these and the cost of buying them is too much for occasional use, many large cities have buffing or polishing services listed in the phone book and smaller towns might have a machine shop that has the equipment.