Fungus
I don't know who put 'fun' in fungus, but it's certainly no fun to remove it. Fungus mainly grows on matt painted surfaces, when the F2 was stored in a damp or humid place. Removing fungus is a dirty, tedious and time consuming job but thankfully only about 5% of F2s I received had fungus infestations. Below are some images of 'before' and 'after' my fungus removal treatments.

To prevent fungus growing in your F2, always store it in a sealed bag with a moisture absorbing packet inside. I used two sealable sandwich bags per camera, and that had worked well in Sydney Australia. Also, to avoid condensation forming inside the F2, put the camera in a sealed bag before taking it from a cool/cold place into a warm and humid place.

When you buy an used F2, always carefully smell and check for signs of fungus or corrosion, as it's expensive to remove them. Avoid buying F2s from humid countries/states, as it's almost certain that they would have some fungus inside them. If you had received such a camera, then send it back for a refund because fungus spores can travel to your other equipment and germinate when the ambient condition is right for them to grow.
Mirror housing before treatment Mirror housing after treatment
Mirror holder before treatment Mirror holder after treatment
Eye piece lens and foam before treatment Eye piece lens after treatment and foam replaced
Finder housing before treatment Finder housing after treatment