The scaling that you experienced on the most recent placement is the result of a water-cement ratio that's way too large, in the upper surface of your pour. And you're right, it was probably caused by working it too soon, instead of allowing the bleed water to first evaporate off. Unfortunately, there's not much that can be done to correct it, other than removal and replacement.
It's also possible that you are simply pouring too wet in the first place, instead of going with a stiffer mix that would result in less bleed water. A 3" slump is close to ideal, and is indicated by gobs of concrete "standing up" on a flat shovel instead of flowing off and flattening. You know you're pouring too wet if, when you spade the mix to consolidate it, it immediately closes up around the spade marks instead of them staying open.
For interior placements like yours, it's often a good practice to set up a fan blowing air across the finished surface, and turn up the heat, all in an effort to hasten evaporation of bleed water. Another option, although I've only done it a few times, is to broadcast dry Portland cement uniformly across the wet surface and then float it in with a hand (resin or mag) float. Doing so enables steel troweling to be done sooner, but there's a possible drawback of over-working the finish and thus also weakening it.