I like to use 800 wet sand if I'm spraing metallics without a sealer. 400-600 works well on solid colors or single stage. I'll back up what Pat said. Don't sand base coat before applying clear. you can sand out little nibs of trash but you have to apply more base over the area before applying the clear. You will be able to see the cratches if you don't. I like to clear over single stage also. I like to let cure, sand it with 1000 or maybe 2000 if there's not a lot of texture, then clear over it. Just keep in mind you are putting a lot of material on the surface so don't get too crazy with the number of coats. You don't want it too thick. We did a Black '40 Ford that way and the clear layed down so flat we only had to spot buff a couple of trash spots and the car looked awesome. The customer was well pleased and the car had nearly no texture so no reason to buff it.