What determines if a plaster can be floated or can only be sanded as the final smoothing step
Yeah, after trying this in practice, this floating 101: you can't bring out aggregate that's much finer than your float's sponge. The max 200um stuff turned out to be some lawyerly spec (as in them not getting sued if someone finds a spec of sand in it); in practice it's as fine a flour or even finer (perhaps close to toner dust). There isn't any float in the typical hardware store that has a sponge that fine. You can "float" it of sorts, but you're not really bringing out the aggregate, just leaving impressions that match the sponge's pattern. You can surely do interesting decoration this way, but it ain't floating.