I was having a very difficult time with a few darkslides on several older holders, of various sizes. In desperation I looked thru my stock of stuff, and found a can of Johnson's Paste Wax. I tried the wax on the slides rubbing it in well both sides, then buffing. Man, they work like new. Just passing this on. And I should add, this was on dark slides that were so tight it could disturb the camera when pulling them out, even if you were holding on to the camera.