So I'm cobbling together two old Nikon FEs (my favorite!), and they've both been living in grunge-ville for decades. Both bodies are essentially parts bodies, and I have already gotten my value from their lenses or other components of a batch buy. So I have nothing to lose! I slammed all the best parts into a single body, did a whole lot of repair, and I'm staring through the viewfinder at a really ugly dirty mess. What to do? Of course I removed the focusing screen and cleaned it properly. I used a rocket blower on the mirror and the underside of the prism. Still a huge amount of grime. Next, I went after the prism with Residual Oil Remover on a Q-tip, a trick I've used a few times to good effect. Improvement, but still foggy / grungy / dusty / speckled. Okay, it's that danged mirror. In spite of all advice to the contrary, I'm going for it. I soak a qtip in ROR, and gently swab, moving fresh product and cotton threads into use for each area of the mirror. Then, I swabbed clean with a different qtip, pretty aggressively! Now, everything I've ever read says NEVER DO THIS! Touching the mirror is a death sentence! And, well, I don't recommend it for your high-class SLRs. In fact, I'll tell anyone who will listen that this is not an approved method, and may ruin your equipment for life! So don't come a blaming me later! You've been warned! But you know what? Heee heee! Good as new. (Or so close, you'd be very happy anyhow). Shhhhhhhh!!!!!!! Don't tell on me.