So I just went through the same thing with my XD11. I am fortunate enough to know the history of the camera, as this was my dad's old camera.
First off, the side plates are not metal, so please don't sand them! Also, the white caps are more of a powder, so please do not just paint over them! I'm not sure what material they are, but they are glued/stuck on rubber or plastic. If you look at the diagram in the service manual (which you can Google search) you can see for yourself. After staring at the camera and the manual for a while, it finally became apparent to me that this was not metal, and eased my mind on the next steps to take.
I had assumed the white caps were salt deposits (as I know my dad used this around the ocean a lot), but I hadn't considered it may be mold! This is why I say to not paint over it. Mold spores don't just die if you paint over them. Eventually they will show again.
What I first did was wipe with a water moistened cloth. Afterwards, I used new cloth to wipe with 99% isopropyl alcohol. After several passes, the white was completely gone. Use a clean cloth for each pass. You will know if you got it all after it completely dries. After the second and third pass it looked clean at first, but after time it turned a cloudy white...because I was reusing the same cloth. Also, don't use cotton swabs. You will get a million tiny fibers left behind on the raised bumps. I'm fairly certain those bumps are manufactured. My Pentax 35mm's use the same material, and they are bumpy as well, but never got the white stuff.
If you're wondering how the mirror is locked up, I just used bulb mode to hold it up. You can easily lift the mirror and hold it up by hand too.
I hope this helps. Take care