I've been struggling with my seat swiveling for a long time, and now I have suspicions that the seat might be sinking. The problem appears to be the collar that sits between the seatpost and the frame. I'm reluctant to crank to QR really really tight as I don't want to damage anything. The bike is old, so is this a corrosion thing? We have some 'shim in a can' at work, would that help?