OK, this sanding thing is completely new now.
Before, when I was using the brush, I'd be sanding down drips and bulges around the edges of the panels, and hoping to even out ridges in the brush strokes.
Now that I've sanded my first coat of enamel laid down with the roller, though, it feels totally different. The amount of paint that goes on seems like much less--there are certainly no excess drips over the edges. And the slightly eggshell-y, all-over surface levels out a bit, but not completely when it's dry.
But the big difference are these tiny bubbles, which end up sanding right out, giving the whole surface a pretty smooth touch.
It'd be easier if there were no bubbles, of course. I'd love to paint a coat, have it dry, and see that it's finally the perfect, featureless, skin-like smoothness I want. But the bubbles showed up again in the new coat [above]. Maybe I'm shaking the paint too much, or not letting it sit long enough, when I open it? But such bubbles couldn't transfer from the can to the sponge. I suspect they're coming from the roller, which is probably not saturated enough.
Anyway, it's working. Which is nice.