In their natural state, cast aluminium parts are relatively rough, with a flat silver finish. Aluminium parts, whether they're on a car, a motorcycle or something else entirely, can benefit greatly from polishing. Polished aluminium can have a deep, mirror-like shine and looks far better then when the parts are first cast. Though there are businesses that specialise in polishing, this is also something an amateur can do with the right approach.
Remove the aluminium component from wherever it is installed. The polishing process involves power tools that spin very quickly and the polish can be messy, so it's best to do the polishing away from where the part is installed.
Place the part in a vice or clamp it down somehow. Begin the polishing process with a 40-grit 5-inch sanding disc mounted on the power drill. Sand down the casting marks and any rough edges that you want removed. Be careful not to gouge the metal since aluminium is relatively soft and the 40-grit is rough.
Rinse the part off with water. Sand it with the 80-grit sanding disc and then the 180-grit disc. Alternate between the two if there are deeper scratches that the 180-grit won't remove. Sand hard-to-reach areas with conical sanding attachments mounted on an air-powered die grinder or on the power drill.
Rinse the part with water. Sand the part by hand with 120-, 180-, 240-, 320-, 420- and then 600-grit wet sandpaper. Place the sandpaper on a foam sanding block or pad. Keep the aluminium part and the sandpaper wet for this entire process to minimise scratching.
Place a 6-inch polishing pad on the power drill and apply polishing rouge to the buffing wheel. Polish the part, taking care not to press too hard and work the pad consistently and evenly until the part begins to polish. Work on small areas at a time and stop and check your progress frequently. To keep the rouge soft while you polish, it is recommended that the polishing be done in a warm place.
Polish hard-to-reach areas with a small, cone-shaped felt polishing pad. When all of the desired surfaces are polished, wipe the part down with a clean microfiber towel.
Finish the polishing with a new clean pad and switch to the less abrasive, white polishing compound. This should bring out a final mirror-like shine on the part. Wipe the part down with a microfiber cloth when you are done.