When I'm applying scenic grass or dirt to a diorama, I sprinkle on generously so I can pat it down into the glued surface. Then after a night of drying, I'll us a paint brush on the excess to break it up and follow up with a dust buster that has a nylon stocking over the end to suck up the extra. The Stocking keeps it all together so that I can reuse it. If for some reason, there is a mixture of grass and dirt together, I save in a seperate container to use on dio's that the landscape isn't so critical on.
If you're using a knife of any sort to trim/cut parts and the part is so small you feel that you absolutely have to hold it in you hand, for gosh sakes, where leather gloves. Even better would be knife proof gloves that many fisherman use. One day I was cutting a small part and while I was cutting I was thinking to myself, "this is du..." That was as far as I got before I sliced of a hunk of my right index finger. It healed, but the scar is still there and shows no sign of ever going away.
Don