No, this isn't a scene from Scarface.

twinrider1

I've been learning how to mud. Removed some chair-rail height paneling to find a big gap, a foot off the ground, horizontally all the way around the living room. 9' ceiling....8'drywall with a 1' filler.

I know I'm using too much joint compound. There's enough mud dust on the carpet to keep Tony Montana busy for a week.
But the end result is actually starting to look pretty good. If it didn't look good, I'd just add another coat and sand again. I think I'm getting a feel for it.

How smooth is does it need to be? Will primer help level things out, or does it tend to magnify problems? I'm using SW Multipurpose primer because the top half of the wall is painted and the lower half is bare drywall.

havalife

Don't count on primer helping you. If you can feel anything when you rub your hand on it then you need to work it a little more. Before you paint or primer take a flood light and use it to look down the wall this will help you find imperfections. Once you prime drywall mud it's a pain to sand take your time and get it right.