Just clean and rinse very well. Kitchen dish soap and a sponge is good enough for me.
No primer is involved. Paints and stains are often considered to be "penetrating" or "film-forming" depending on whether they lay on the surface. A primer is made to bond to the surface, and then a film-forming layer bonds to the primer.
This car stuff is made to bond directly to plastic, hence the name.
I laid out newspapers on the front porch and gave it very light coats, from all 4 directions, and only needed to wait 20 minutes between coats.
Masking the label is a little tricky. I use masking tape and an Xacto knife. I try to keep one tiny corner from bonding down so that I have a starting place to peel it back up. If it is just embossed plastic and not an applied label, you are home free.
Don't mess up any slots or clips that hold the case together when/if you are painting the inside surfaces, I don't know how those are assembled. If it has a detachable cord, make sure to take care around that part.