Absolutely-you just gotta know the right tricks to avoid lumps or streaks! I use a filling knife for wood filler all the time (fixed my old wooden chair last month, and it looks brand new now). Here's how to do it smoothly.
First, pick the right knife: a 3-4 inch flexible stainless steel one is perfect. The flexibility helps the blade follow the wood's grain-super important 'cause wood isn't perfectly flat, and you don't want to miss little grooves. I tried a stiff knife once, and it just pushed the filler around instead of working it into the wood-total mess.
Next, prep the wood: sand the area lightly first (fine-grit sandpaper works) to smooth out rough spots. Wipe off the dust-filler sticks way better to clean wood. Then, scoop a tiny bit of wood filler onto the knife-don't overload it! Too much filler makes it hard to spread evenly.
Hold the knife at a 30-45 degree angle to the wood, and spread the filler in small, smooth strokes. Follow the grain of the wood-this helps the filler blend in so it doesn't stand out later. If you're filling a hole, press gently to work the filler into every nook-no air bubbles allowed! Then, use the edge of the knife to scrape off excess filler-hold it almost flat now, and swipe in one direction (with the grain) to get a smooth surface.
Pro tip: Wipe the knife with a damp rag every few strokes-wood filler dries fast, and dried filler on the blade will leave streaks. Let the filler dry completely, then sand it lightly (again, with the grain) to make it perfectly smooth. I promise, once you get the hang of it, your wood projects will look like a pro did 'em!
