Aluminum siding is virtually indestructible, but the coat of paint that separates it from the elements isn't. When that paint begins to peel and turn chalky, the siding looks old and worn. It's definitely possible to repaint aluminum, and as when repainting any surface, preparation is key. Because of its flexibility, latex paint is recommended for aluminum, but the primer should be oil-based; latex primer creates gases that can cause bubbles and premature paint failure.
The layer of chalky paint that makes the siding look old has to go before you can repaint -- if you aren't diligent about this, the chalk will prevent the new paint from adhering. If you choose to scrub it, scrape off peeling paint first, using a paint scraper. Mix a strong soap solution consisting of 1/2 cup of trisodium phosphate per gallon of water for washing and scrubbing. Attach a scrub brush to an extension pole and use this to wash and scrub, then rinse with a garden hose.
Work Faster with a Power Washer
It can take considerable time to scrub an entire house, and if you'd rather spend that time on the actual paint job, rent a power washer to remove peeling and chalky paint. Ideally, the power washer should be capable of developing 2,500 pounds of pressure, but you can do the job even with an electric model that develops 2,000 pounds or less.
Priming and Painting
You should prime any areas of exposed metal before painting, but if your siding hasn't been painted in a while, an undercoat of primer over the whole house can only be a benefit. It's important to use an oil-based primer, because latex paint contains ammonia, which reacts with aluminum to produce gas. It isn't enough gas to create a hazard, but it does interfere with adhesion. One hundred percent acrylic latex is the best choice for the top coats, on the other hand, because it remains elastic and resists cracking.
Painting with a Brush and Roller
Cover the ground and surrounding bushes with drop cloths, and mask any parts of the house you don't want to paint, using masking paper and painter's tape.
Paint the edges of the wall, as well as the areas around windows and doors and under the roof overhangs, with a paintbrush. Use a natural bristle brush to spread oil-based primer and a synthetic-bristle brush for water-based paint. Natural bristles spread paint more evenly, but they become flaccid when used with water-based paints.
Fit a roller with a medium-nap cover, load it with paint and roll parallel to the slat direction, starting at the top of the house and working down. The roller will probably miss places, especially near the seams between panels, so use a paint brush to go over these areas.
Let the primer or paint dry for the recommended time -- usually two hours -- then apply the next coat. Most jobs require one coat of primer and two topcoats.
Painting with an Airless Sprayer
You can save time by painting aluminum siding with an airless sprayer, and because you won't see any brush or roller marks, the results are more attractive.
Set up scaffolding around the house or work from a tall stepladder. If you have to use an extension ladder, secure the top to the fascia with hooks and ropes to prevent the ladder from moving. An airless sprayer generates enough backward force -- when you press the trigger -- to move an unsecured ladder.
Start painting at the top and work your way down. Adjust the nozzle to spray a pattern perpendicular to the panel direction, and move the sprayer parallel to the panels.
Keep a paintbrush handy to brush off any drips you see developing on the edges of the panels. It's best to get these while they are wet, but if you miss one or two, you can always sand it flat and touch it up when it dries.