Originally posted by excanonfd Depends on what you fouled it with
Sounds like it was fouled with rotten foam goo. That stuff is VERY hard to remove. Touch it and it smears into the grooves of the Fresnel lens surface. A few years ago I had this happen and truly made a mess of trying to get it clean. The difficult thing is that the screen is made of a clear (acrylic?) plastic material and may be attacked by solvents. I tried Windex (glass cleaner), but that just spread the mess. The camera took a trip to the service shop and $40 USD later the mirror, screen, prism, and ocular were in like-new condition. I asked the tech what he used and he winked and said "secret formula".
This last spring, I got some crumbs on the focus screen of an old Spotty and was able to gently "lift" them off using a fiber "applicator stick" (like a Q-Tip cotton bud, but much more tightly wound and with a wooden stick). I bought a good supply of these from Micro Tools for cleaning hard-to-reach places.
Good luck.
Steve