Replace the light seals of your good old film camera

Step 3 Clean the mirror box area.

With a Canon F-1, this step is much easier to accomplish, because it has a detachable viewfinder prism. There is virtually no risk of contaminating the focusing screen (which is so fragile). If you are working on a camera with fixed prism, be very careful and do not ruin the focusing screen (it cannot be wiped, cleaned, and it is easily marred).

Be careful and do not touch on the mirror. It can be stained/damaged very easily.