Welcome to the forum. Glad to have another experienced artist around to throw questions at!
In my opinion a primer coat *IS* the best way to find imperfections. I sand until I think things are smooth as glass and a coat of primer always shows how wrong I was. Sand a little more, re-prime the sanded areas, and try again.
You can get very fine grit sandpaper, up to around 1000 grit, at stores that sell auto body repair materials. NAPA Auto Parts around here has 700 grit, and that's what I usually use.
I'm not an artist so I'm not familiar with the material you are using, however cleaning the surface prior to priming is always a good idea. For plastic models I usually use alcohol since that will remove the mold release lubricant, and then blow everything off with high-pressure air from my compressor. Most people just wash them with water and let them air dry thoroughly in a dust free area. A tack cloth is also a good idea.