Yes, you can use a taping knife for plaster-but there are a few things you need to know first. Plaster is thicker and stickier than drywall mud, so it's a little trickier to work with. Let me share what I've learned from patching plaster walls in my old house.
First, size: Stick to 8-inch or 10-inch taping knives for plaster. Plaster is thicker, so a smaller knife (like 6-inch) will be too hard to spread-it'll just glob up. A larger knife (12-inch) is too wide for most plaster jobs (since plaster patching is usually small, like fixing cracks or holes). I use a 10-inch knife for plaster-it's wide enough to spread the plaster evenly, but small enough to control.
Next, blade stiffness: Stainless steel is better for plaster than carbon steel. Plaster is thick, so you need a stiff blade to spread it without bending. Carbon steel blades are flexible, which is great for drywall mud, but they'll bend when you press them into plaster-leaving uneven patches. Stainless steel blades hold their shape, so you can spread the plaster smoothly and get a flat finish. I tried using a carbon steel knife for plaster once, and the blade bent, so the patch was bumpy. Had to sand it down a lot.
Pro tips: First, wet the taping knife blade slightly before using it with plaster-this prevents the plaster from sticking to the blade. Second, spread the plaster in thin layers (plaster dries fast, so thick layers will crack). Use the knife to smooth the plaster, feathering the edges so it blends into the wall. And clean the knife immediately after use-plaster dries rock-hard in minutes, and if it dries on the blade, it's almost impossible to get off.
But here's the thing: If you're doing a lot of plaster work (like resurfacing a whole wall), a dedicated plastering trowel is better-it's curved to hold more plaster and has a smoother edge. But for small patching jobs? A taping knife works just fine. I've fixed dozens of small cracks in my plaster walls with a 10-inch stainless steel taping knife, and you can't even tell they were there after painting.
