I want to paint my PS1 primer gray. How important is it to sand between coats and if so, what grit sould I use.
Isn't there vinyl dye in that color? It's harder to use, but since it's dye and not paint, there's no sanding involved. I suppose most vinyl dyes are glossy and not matte, which is what you're aiming for. The grit of the sandpaper is less important than using a good block. Since your base material is plastic and not wood or metal, you probably want a nice flexible block. I think 3M makes a 300 grit block that's already got a sanding surface attached, so you don't have to affix or remove paper as a separate part.
I always just use my best judgement. I make sure to sand and clean before beginning painting, but in-between coats I just make sure it's a good consistency and that there are no artifacts in the coat (stray hair, landed bug, etc.). I've used vinyl dye once, and if I would've done it on a surface that hadn't already been painted I think it would've turned out better. It struck me as being far easier to use than spray paint, since it doesn't run and a clear coat probably isn't necessary, although I would still recommend one.
VHT make a matt grey vinyl dye, I used when doing the xbox slim mod. Here's the site I got it from: https://www.usautomotive.co.uk/store/scripts/prodList.asp?idCategory=59 UK supplier, so it may not be of use to you (i'm guessing you're in the US as you spell grey the wrong way :lol Another point, paint will fill in small scratches & scuffs, vinyl dye won't.
get case off the internals. soak in the water with detergent and after few hours give it thorough scrubbing. go out and get the automobile primer - Duplicoat(?) seems to be the choice... spray the case. after overnight drying time wet sand the surface. after washing and drying do another coat of primer. dry overnight. did another wet sanding - I believe you know how to do wet sanding? check the surface and if not satisfied do the above steps. after the surface is smooth and clean seal it with the top coat - probably use the same brand as the primer. *Primer usually comes in the gray so I didn't mention of painting stage. If you want a particular gray, get the spray can of that colour and after wet sanding the initial primer coat paint the case with your choice of colour. After drying do the wet sanding followed by drying /another coat of colour/wet sanding. After that if you are happy with the result seal the surface.
