Clean the siding to remove grime and surface oil. Use either a commercial cleaner formulated for metal and galvanized steel, or an ammonia-based cleaner.
Sand the siding lightly if installed for less than a year. Using an electric palm sander with 80-grit sandpaper will speed the process. Skip this step if the siding has been exposed to weather for a year or more.
Use a wire brush or drill with a wire brush attachment to remove rust. If there are patches of rust you cannot remove, coat them with rust converter and allow to dry for 24 hours before priming.
Prime the metal with a rust-inhibiting water- or oil-based primer. An oil-based primer may be more effective, especially if you apply two coats. Allow to dry according to label directions but do not leave metal unpainted for more than 30 days.
Paint the siding with two coats of very good quality 100 percent acrylic (water-based) or alkyd (oil-based) paint. Spraying the paint will give the most uniform, glossy finish if you're using a satin or semi-gloss paint. Paint can also be applied with a roller and brush.