Finishing an aluminum die casting

How can I get a smooth finishing from a die cast aluminum part that has to be sanded after die casting? I've tried ball burnishing but it only seems to "show off" the sanding imperfections.

Thanks

Surface finishing is a negative process of material removal. If you don't improve the surface before polishing or sealing, it will only give you an existing surface profile of the material you are working; therefore, to improve the surface and/or blend in the rough profile, you need to remove the irregularities (peaks and valleys). This is done via a number of methods. In your case, you need a coarse and maybe finer forms of an abrasive preform ceramic media. For non-ferrous you should consider plastic media shapes. There are no short cuts. You need at least a 2 step process, maybe 3.