I would have to agree with Paul - I chose not to add a finish to my polished aluminum. I have a few reasons for this. The problem I had with putting on a finish is that when it scratches, I have to take the trim off, remove the finish, polish it, put the finish back on, and install the trim back on again.(Of course this is not practical with some pieces). It then rarely matches the rest of the aluminum. It is much easier to touch up aluminum that has not been coated. I know they sell polish/wax that protects the polished aluminum that might be more practical. Aluminum does oxidize quickly but the resulting aluminum oxide forms a tight barrier against further oxidation. Even though it does not have a high polished chrome look for very long, people that have seen my car still complement me on the results. The bottom line is that personally I like the look without any finish.
For what it is worth, I polishe the aluminum myself. I had never done buffing before. Eastwood was having a sale on supplies and a "how to" video. At the time Grizzly was also having a sale on one of its buffers. For less than the price to get my aluminum buffed, I was able to purchase the buffer, video, and supplies. Now I have the equipment to do this anytime I want. I admit it is very time consuming. There is a lot of prep work first before buffing. I like to learn how to do things myself on this car and it was rewarding to do this myself.
Kevin