First: I'm using plaster of Paris. Here's the thing - if there's prehardened plaster (i.e. something with crystals already in it) in your bucket? The crystals will catalyze the new plaster into hardening really, really fast. So I mixed up just a little - and it was too hard to apply within five minutes, I kid you not. And that doesn't leave much time for smoothing. Of course, on that wall, any amount of smoothing is an improvement, and nobody says I can't take several stabs at these patchy bits, either.
After seeing how well that didn't work (well, it was exciting to try my new big steel trowel, and it was my first top coat attempt, and that was fun) I went back to the Google drawing board. The chaps over at diyplastering.co.uk have some nice information about plastering, and bucked up my courage in thinking maybe this is something I can manage. And the guy at brooklynrowhouse.com swears by his Magic Trowel, so maybe I'll try that - but he also adds a little vinegar to his plaster of Paris to keep it softer longer. That's a good idea.
But one thing everybody says - keep your bucket and tools as clean as humanly possible. I didn't. For mortar or basecoat it just doesn't matter - but plaster of Paris cares.
More later, after I have time to experiment some more. I'm going to need the practice, because after I started peeling the paint off the walls in the upstairs sunroom, huge patches of the top coat came off, too. So I have most of a wall to do in there. And of course there's lots of top coat repair in the dining room, too, and I'll be seeing any flaws there for years.