Great question-they're mostly reusable, but there are times when tossing one is totally okay! Let's break it down. If you're using it for stuff that's easy to clean-like smoothing painter's tape, scraping sticker residue, or spreading spackle that's still wet-you can absolutely use it again and again. I have a plastic putty knife I've had for almost 3 years now.
Every time I use it, I just wipe it down with a damp cloth right after, and it's good as new. No sticky gunk, no cracks-still works like the day I bought it. Even if you use it for joint compound, as long as you clean it within an hour or two (before the compound dries rock-hard), soak it in warm water for 10 minutes, wipe it off, and it's ready to go.
But here's when disposable makes sense: If you're using it for something super messy that's impossible to clean-like oil-based paint, thick glue, or even moldy caulk. I once used a plastic knife to spread epoxy glue for a DIY shelf, and by the time I was done, the glue was already setting. I soaked it, scraped it, even used rubbing alcohol-nothing worked. It was a sticky, crusty mess, so I just tossed it. And since plastic putty knives are cheap (usually
2−5 each), it's not a big loss.
Also, if you're dealing with something gross or toxic-like lead paint or mold-tossing it after use is smart. You don't wanna risk spreading that stuff to other projects. So yeah, reusable when you can, disposable when you need to-no need to overcomplicate it!
