I think [member=22025]overanalyze[/member] has it right for this scenario.
I just redid a closet and an accent wall this past week. The closet needed a lot of work, so I skimmed the walls and ceiling with 90, 45, and then touched up with all purpose. It took a while and honestly, cleaning out the buckets can be a drag when it's 40 degrees outside.
The accent wall just had a lot of texture from years of repaints, so I used all purpose. I didn't prime, but I knew the wall had been repainted two years ago with an acrylic paint. I don't think my mud so much, but similar to [member=22025]overanalyze[/member] , I work a couple feet at a time, applying the compound and then knifing it flat. It moves pretty quickly once you get into a rhythm. I can't scrape as well with a trowel as a knife, so I will trowel on and knife off. Make sure to slightly favor the wet side of the trowel so that you don't drag through where you just smoothed.
The other thing is, make sure you seal your skim coat well once it's applied. I'm still a novice, but I've used Zinsser123 and Primecoat in the past. This time, I put on some BM Super Spec Masonry sealer... I think it should equalize everything.
-Adam