Why would you fill a keyhole with dust? Once the keyhole is full of dust, where will the key go? I have had to repair many a lock that had years' worth of graphite buildup inside them. Better to use a non-oil based spray (such as WD40) than to fill the keyhole with what amounts to pebbles. Remember, those graphite particles are crystals and will scratch the tumblers, eventually wearing them down. To you and me, with big clumsy fingers, graphite is smooth, but to tumblers measured in thousandths of an inch, those crystals are pretty big. Never use graphite. Never use oil. Use a spray that will lubricate and flow out.
