i have a 40+ Nikon SLR and would like to clean the mirror where some of the bumper has transferred to the edge of the mirror surface. I don't think it effects the metering but worried it will effect the new bumper. I have heard it is a front surface mirror and should not be touched. But the mirror looks great after all these years which leads me to think the mirror has a overcoat to protect it from the environment. What should I do...leave it alone or clean it? If cleaning is the route what should I use and how should I do it?
I would only clean the mirror if it is necessary, i.e. if it is clearly dirty. Sticky residue from the dampener is one of them.
Isopropyl alcohol would be the gentlest way to start, then lighter fluid and acetone. The latter would be my choice as it dissolves best sticky residues of rotted foam.
Dip a cotton swab in it and carefully move it over the mirror without applying pressure. Don't aim for perfection, this might save you scratches.
But in any case first I would test a corner of the mirror to see if the acetone damages the coating of the mirror.
So my recommendation is expressly with reservations. Mirrors are easily damaged.
If in doubt, I would give it to a professional, especially since it is a valuable camera.