If you don't think the rosin is producing any "dust", take a key, sandpaper, a coin, or any other sharp object, and lightly scratch the rosin. You will see some light streaks if you scratched hard enough. Too much rosin will cause the bow to grip too well, producing a scratchy sound. If you over-rosin your bow, it's fine; it'll just take a few hours of playing to bring it back down to the correct level. If this is a newly haired bow, it may need more rosin than normal. Draw the flat side of the bow hair across a string to see if it makes a clear sound after three or four strokes of rosin. If it doesn't, add a couple more.
