In the best case scenario, drywall knives get cleaned pretty much as soon as all of the mudding work is done. Drywall mud is (obviously) much easier to clean up before it dries and hardens. But sometimes life gets in the way and the perfect scenario just doesn’t happen. Don’t freak out if you end up with knives that are so coated in caked-on mud that they’ve become practically useless. Instead, take out your favorite drywall sander and get to work. Pick a grit around 120 and just sand off the dried-on mud, same as sanding the mud off of a wall. Just make sure you aren’t too aggressive and end up damaging the blade.