Cool, sounds like a good plan, but alot of sanding between coats of paint. I also just used rattle can spray paint for my box too, but it was lacquer spray paint, not sure if it sands differently then the paint your using....
I sanded my bare wood box with 80grit too then used a "polyethylene wood sanding primer" that I brushed on 3 coats thick. I sanded the primer with 150-200 then just sprayed on 4-5 coats of black lacquer (5 cans total over 2 days)
without sanding between coats. After the paint cured for a few days I then wetsanded the entire box with 2000 grit to get this finish...
Although I have read the way that your doing it by sanding in between coats is the right way, I just skipped doing this on my box because the paint felt soft to my touch and I didn't feel like sanding between coats was the right thing to do because I could feel it ball up under the sandpaper, which is why I waited till the end of my painting to wetsand.
Hope this helps, good luck.