Hi There: There are others who can answer this better than me, but I'll give it a shot. For one, you can't 'completely' clean a slide while it's in the mount. You'll end up with dirt around the edges of the mount. It depends on how dirty they are, and what kind(s) of dirt you're dealing with. If it's just dust, try using a brush - similar to one you'd use to clean your camera, to brush the dust away. Be extra careful on the emulsion side. It's generally suggested not to clean your slides with water, the reason being that as it dries water can leave behind water spots. If you do use water, make sure it's distilled. Better is to use a film cleaner - this should be easy to find at a good camera store, one that sells darkroom equipment. I always kept the plastic slide mounts from slides I tossed, and I have no reservation in reusing them.