I found a chemical that works good, but it is dangerous. There is a product called "Sabre" sold by ecochem.com. This is a sulfuric/hydrochloric acid solution and it is used to clean aluminum tanker trucks and trailers. You have to mix it down. I put the solution in a small spray bottle and wet the item down and let is sit for 5 minutes.

I cleaned all of my light castings with it and they look brand new now. You have to clean the parts after with soapy water. It is a good idea to wear gloves & goggles too as you will get chemical burns if this stuff gets on you. (just ask)

Really good results. Don't use it on chrome or it will remove it.

Also, the acid takes off the reflective paint inside so you will have to respray the reflector with reflective aluminum paint. (Which is the way the factory did it) I masked mine better.