I've just been through this.
Before you start, be aware that the N89 alloys are factory coated and you will need to remove that coating in its entirety in order to achieve a satisfactory result.
You can:
1. Strip the coating (Klean-Strip EAR322).
2. Spray on an etching wheel cleaner (Eagle One 665804) or dip the entire wheel in a stripping solution (Streak Master Aluminum Brightener). You will need suitable gloves and eye protection and if you dip the entire wheel a good sized (22-1/8" D X 16=1/2" H) rope handle tub. Be careful not to overdo this step as if the wheel is made porous you're pretty much screwed.
3. Hand sand the machined surfaces with progressively finer grits of sandpaper, starting with a grit aggressive enough to remove the worst scratches and finishing with perhaps 2000 or even 2500 grit.
4. Lastly, have someone (hopefully local to you and of high quality) powercoat the wheel.
The above is a messy, labor intensive, giant PITA, as well as a chemical nightmare. Figure on about $100-$200 plus the cost of powercoating. Of course, none of the above is useful if the wheel is bent or otherwise damaged.
Alternatively, you can ship your wheels to Detroit Vintage Wheel and they will restore them to show quality. Figure on about $800 including shipping to and from.
This is the dilemma with unrestored N89s and why so many of them are for sale so cheaply.