Hello,
Just curious if anyone who has modified plastic cases would recommend a sandpaper grit (or set of grits) that are good for cleaning plastics? I am just looking to clean and make the inside of a ABS case smooth.
I plan on using if for a mod on a old Gameboy Pocket, but I figured this was as good of a community to ask. Thank you! 
Tl:dr, all of them.
Sanding is always a multi-stage process. Coarser grits remove more material but leave a rough finish, finer grits remove less but the finish is cleaner. For proper use you always want to move coarse to fine, as the finer grits will clean up after the coarse ones before it.
For molded plastics I’d say start at 120grit and see how that does then move up from there, probably max out around 1k depending on how the finish on the rest of the area is.
Micro Mesh is an example of a brand who also makes grits up in the 10K+ range, which would allow you to sand back to a completely clear finish.
Even the white scotch pad in my Norbauer polycarb refinish pack was not fine enough to make my plastic clear and shiny. I suspect you would have to use a polishing compound and a lot of elbow grease 
Thank you everyone! Luckily my modifications are on the inside of the case. It doesn’t need to look perfect but I wanted it some what clean.
I will try the different grits!
lower than 220 for reshaping plastics, removing deep textures and stuff, Up to 800 for a matte finish, anything over that will start brining out a nice luster, as mentioned above, micromesh is a thing, commonly used by Guitar Luthiers to polish the clear coat on guitars, or by me back in the day to sharpen stuff 
. then plastic polishing compound to bring a gloss to it… vapour polishing for a flawless shine (With certain plastics like PC and ABS)
You can also use polishing papers as your last steps instead of traditional car sand papers. They’re more appropriate for plastic keyboard cases. It’s also what I use for my LEGO keycaps.
How about some mr clean magic erasers?
aka Melamine sponges?
Supposedly when they get wet they are equal to up to 3000 grit sandpaper. Might be too fine
Thanks for the tip! I wasn’t familiar with these so I will check them out. 
Good idea! I will be using some nippers to remove plastic, but that is only going to get so close…
I think I want some chunky sandpaper to remove more material. Luckily all of this is on the inside of the case!