I tested various rollers and brushes. Have to say for the most smooth finish a high density foam roller did the best AS long as you don't go over the same spot more than twice (two strokes) before dipping it back into the paint. *shrug* You'll get a hammered, spotted look if you do more before dipping.
Thin it. So they say. I did tests with 2.1 and 1.1 and no thinner. Frankly, I like the non-thinned the best but the auto painting non-pro guides I've found say it has to be thinned. On flat surfaces--non-thinned is fantastic with a fine hair brush used. Deep, even, just gorgeous. But get a little thing like a rivet and it's a clump and runny mess.
However, rivets will be your bane, roller or not. I started brushing riveted areas with a light coat first then hours later did a little roller touch up. alksdjflkajdf things. Rivets make for runs and hard to get even places.
If I had to do it again (and I do) I'd buy or rent a sprayer.
I've lots of wet sanding to do ALL over the place. The worst is the back where I was testing out the non-high density rollers. That just looks hidious beyond belief.