4 stages for degreasing and polishing aluminium rivet heads

Hi .I have a production company in Iran producing aluminium blind rivets . The aluminium rivet heads are greasy and must be washed. I use a through vibratory machine for washing and at the same time deburring and polishing them .I wash them in 4 stages . First I fill up the vibrator with hot water and as the machine is ON and the rivet heads are rubbed against each other I add sodium carbonate ⇦ on eBay or Amazon] . After 5-10 minutes the water is drained and the machine is again filled with hot water and again I use weak alkaline detergent for 10-15 minutes. again the water is drained and this time I use sulfonic acid and leave the vibrator ON for about 45-60 minutes for the rivets to be polished.After this stage the rivets are really shiny but the final stage is what I have problem with. How to get rid of the oxide that remains on the surface of the rivet . I've tried sulfuric acid and acidic detergents but still after rising the rivet heads leave a dark ,gray, shiny stain when rubbed on a piece of white sheet.May be I need to add some kind of chemical in my fourth stage that coats the rivet heads. What do you suggest ?