Corners and edges are tricky-you need a knife that can get into tight spots without making a mess. From what I've found, the best ones are small, flexible knives-1-2 inches wide, with a slightly angled blade. Let me explain why.
First, size: A 1-2 inch blade is narrow enough to fit into corners (like where the wall meets the ceiling) or along edges (like baseboards). I tried using a 4-inch knife on a wall-ceiling corner once, and I ended up spreading filler all over the ceiling-total disaster. The small blade lets you be precise, so you only put filler where you need it.
Then, flexibility: A flexible blade can follow the angle of the corner without bending too much. Corners aren't always perfectly straight, so a stiff blade will leave gaps or lumps. Look for a knife with a thin, stainless steel blade-it'll bend just enough to get into the corner, but still be sturdy enough to spread filler evenly.
I also love knives with a "corner tip"-some brands make filling knives with a slightly pointed edge that's shaped to fit into 90-degree corners. I bought one last month for my kitchen corners, and it's a game-changer. No more trying to wiggle a flat blade into the corner-this one slides right in, spreads the filler evenly, and cleans up easy.
Pro tip: When working on edges (like baseboards), hold the knife at a 30-degree angle to the edge-this keeps filler from getting on the floor or the wall next to it.
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