Sanding for a smooth painted finish?

Hello, I'm new to the site. I just got back into model building after a very long time. I pretty much know allot but can always use new methods and ideas to improve. But anyway I just started a Italeri 1:17 scale destert hawk. I am using Testors Model Master spray enamel and I sprayed the cabin parts gray. It came out with great coverage but not perfectly smooth. My question is should I sand it for a smooth flat finish and what grit sandpaper do you I need. I am sure this will apply to when I paint the actual helicopter as well as future kits as well.  I have the Testors sanding sheets. Not sure what color to use. Thanks.