Aluminium wheels scratch and become corroded over tme. This is due to the alloy used to manufacture aluminium, which is soft and susceptible to corrosion. Many high-end wheel makers seal porous aluminium wheels by applying a urethane hard coat onto the surface. This prevents the wheel from becoming dull. You can follow the same process as the high-end wheel manufacturers and clear coat your wheels to make them look like new and keep them shiny for a long time.
Sand the surface of the aluminium wheel using 600-grit sand paper to remove any scratches. This also makes the surface of the aluminium wheel rougher to give the clear coat finish something to adhere to. Make sure you get the sides of the wheel spokes and the backside of the wheel if the spokes have a deep offset.
Wipe the wheel clean using a degreaser compound, white spirit or naphtha. Dry the wheel using a clean towel. Make sure every part of the wheel is free of dirt, brake dust and grease.
Spray a coat of urethane clear coat finish on to the aluminium wheel. You can buy this at most car parts retailers. Every brand of urethane provides instructions on the container for the amount of coats needed for adequate coverage and the amount of time needed for drying. Usually, you need three to five coats with 25 minutes between coats. After that, leave three to five days curing time for the urethane to completely harden.
You may want to remove the wheel covers to access hard-to-reach spots as well as the insides.