katit: I couldn't say how long it took to finish the cover as it wasn't continuous work: to run the project and workshop I make some parts and do the work to order.
Here is a copy of my answer about polishing:
Polishing" is 90% surface preparation and only 10% polishing itself. The better you prepare surface - the better is result. Usually I do it in this way: firstly I remove cast defects, seams and so on where it’s possible. This work I do with help of sandpaper wrapped around rubber blocks, delta sanders (have two of them) and Dremel. Then I sandpaper dry with sandpaper up to 800 grit manually and with delta sanders and Dremel. After dry sending I do wet sending (up to 3000 grit).
Then it’s time for first polishing. I use various headpieces for Dremel for hard to reach places. For other cases I mostly use buffing cloth wheels installed on bench grinder. Once surface polished all defects (traces of sandpaper and so on) became visible. I remove those of them that could be removed and do second polishing.
Some aluminum parts need to be polished long and in all directions to became perfect, while for other parts long polishing makes no good, as it exhibits tiny defects (like pores). Of polishing compounds I use two: mostly liquid Doctor Wax Metal Polish and solid past (supposed to be Diamond polishing compound , dunno of what concentration, but it works just good ).
For those that aren't after a mirror polish look and just want to return their engine covers back to looking like they way they left the factory the following method works fine and is pretty fast.
Use a palm DA sander with 220 grit sandpaper to remove all the corrosion. Small sandpaper rolls on a spinning mandrel can be used in the small nooks and crannies if necessary.
Next is to use a sisal polishing wheel and black cutting compound. I use a cheap car polisher to spin the wheel. The aluminum will get HOT so heavy gloves will be needed. Bear down good and hard and get the wheel cutting. With this step we are trying to remove the sanding scratches made with the 220 and DA.
I usually stop right there, but if you want something brighter you can move to a stitched wheel and polishing rogue to get a brighter shine.
These photos show the finish left after the sisal wheel and black compound. The round portion around the Kawasaki lettering was masked and painted. After the paint dried a sanding block and 320 sandpaper was used to remove the paint on top of the letters.
If you have a lot of time and sandpaper grits running from 36 to 3000, then you can do it by hand.
It just takes a long time.
This rear shock upper mount took a couple of hours.
Finished it with Mother's aluminum polish.