There is a product called liquid steel and I believe they also make one called liquid aluminum. They are sold at hardware stores. They are actually more like a paste. I've not used the aluminum product but I have tried the steel. With the problem on your wheel, I'd run a bead (very small) of the aluminum paste the length of the scratch. Using your fingure if neccessary, make sure the scratch is filled with the paste. After it dries, I'd use a Dremel (or other similar) tool to remove the excess and polish the surface.