Richard, there are several purposes that primer fulfills. The ones that you would probably find most helpful would be high solids primers thatare used to fill small imperfections and scratches. Keeping in mind that the finer grit sandpaper you use, the less primer you will have to use. I like to finish sand to 180 grit and then prime with twocoats of primer,followed by spraying a light coat of a contrastingcolor over the primer as a guide coat. Then I finish sand the guide coated primer with 320 or 400 grit if I ampainting with solid colors, and 500 to 600 gritif I am painting with high metallic colors. Usea medium to soft block ofsome type and sand the guide coat untilthe contrasting color disappears. As you sand you will notice high and low spots (high spots will show primer quickly and low spots will remain colored). Continue to sand until you start to sand through the primer or until all of the contrasting color disappears. I would reprime any areas that I sanded through, as ringlets will form in these areas when you paint if you don't reprime. BTW use the cheapest fast dryspray can paint you can for the guide coat because you are going to sand it all off any way. It's only purpose is to show you any highs, lows, and imperfections you may have missed. I hope this clears up some details for you. I know this can be overwhelming when you first get started, but I have over forty years dealing with painting and paint products so it has become second nature to me. Just keep at it, because in this case practice truly can make perfect.Good luck!

PS- You certainly can use power sanders, but keep in mind you are sanding a relatively soft surface, so go easy and check often.