Troweling up plaster is all about timing-do it too early, and you'll pull the wet mix off the wall; too late, and it'll be too hard to smooth. The sweet spot is when the plaster is in its "plastic" stage-firm enough to hold a shape but still soft enough to be worked. To test, press your finger gently into the plaster: if it leaves a shallow indentation (like pressing into soft cheese) and doesn't stick to your finger, it's ready.
For base coats (the first layer), trowel up when the plaster has lost its shine-usually 20 to 30 minutes after application. This is when it's set enough to hold the trowel's pressure without sagging. For finish coats, wait a bit longer-30 to 45 minutes. You want the plaster to be just starting to firm up, so when you trowel, you're polishing the surface rather than moving large amounts of material.
In hot, dry weather, plaster sets faster, so check it every 10 minutes. In cold or humid conditions, it might take an hour or more. The key is to watch for the "water sheen" to disappear-once the surface looks matte, grab your trowel. Troweling up at the right time ensures a smooth, even finish that bonds tightly to the wall, with no cracks or lumps.
