A friend used XIM clear primer on polished aluminum Mustang wheels and they have looked pretty good for several years. Maybe a slight yellowish over time but still pretty good.
XIM adhesion promoter is like gluing a coating to the substrate, in this case aluminum. The only problem I see is that the XIM will breakdown quickly and lose adhesion. The yellowing you see is the XIM product breaking down. If your buddy got a few years out of that combination he got lucky as it shouldn`t have worked.