I had some alloy castings to clean up that had some corrosion on them. I raided the wifes cleaning cupboard and found a tub of "Pink stuff" that's normally used to clean PVC window frames. Its slightly abrasive and polished up the alloy,removing corrosion quickly. Best of all a large tub was only a quid. By carefully replacing the tub exactly as found it ended up costing me nothing.