A couple of times a year I clean my 5D viewfinder as follows
1. blow out mirror box
2. mirror lock up (sensor clean) blow out mirror box and sensor
3. clean sensor, arctic butterfly then swabs and solution
4. drop mirror and use the previously used sensor swab and solution to clean mirror
5. drop focusing screen, hold edges and blow off both sides, clean both sides with sensor swab and solution
5. blow off bottom of pentaprism, clean with sensor swab and solution (handle of swab is bent so you can reach.
6. reassemble focusing screen
7. install lens
8. clean outer viewfinder with q tip and solution
9. test and repeat steps as necessary
visible dust makes good cleaning solutions
smear away
sensor clean
lens clean
Marc