I forgot you had the high gloss, but the sandpaper in the higher grit ranges(1000+) won't leave much of a mark if any at all, as it shouldn't be moving across the surface, just adds the extra grip you are looking for on the mount.
The lock washer will leave a mark on the high gloss as well as the washer is bigger than the thread opening. A lock washer should be put under the head of the bolt to help apply pressure to the mount which keeps tension on the threads and this won't leave a mark.
You could also get a product known as blue thread locker, used more for automotive purposes and it will keep the threads from turning, it basically glues the bolt to the threads of the threaded insert, but not so much that you can't force it to loosen with the wrench when needed.
