Nikon FM issue with blurred hairlines 'in' viewfinder

I would appreciate any advice to address issue with a Nikon FM.

The camera functions perfectly well but a few weeks ago two blurred lines appeared in the centre viewfinder.
They are less obvious in bright light and do not affect the photographs taken at all- just annoying that they are there.

The lines are symmetrical and do not jiggle when camera is shaken. When I move my eye in the viewfinder to extreme left to right, they change position relative to the focusing screen donut. (see pics)
As the eyepiece, focusing screen and mirror are clean and seals robust, I assume it is the lines are within the pentaprism.
On the FM is is not possible to remove the screen so I have no simple access to check inside the pentaprism cavity.

Short of a total pull apart is there anyway to address the problem?
Thank you
Loos

I would suspect a de-silvering issue with your pentaprism...

https://www.fredmiranda.com/forum/topic/935620

Looks like an identical issue discussed above. It will only get worse if you don't remove the foam touching the prism.


How to remove the top.

My guess too.

Jim B.

Desilvering at the crest of angles of the pentaprism not uncommon. I've seen it with the nikon brand more than any other.

Thank you everyone for your advice.
Looks like it would be impractical to repair.

I removed the prism of an Olympus OM1 that had pretty bad de-silvering, cleaned off the rotting silver with a qtip, masked off the clear surfaces with painter's tape and sprayed the entire prism with 3 light coats of Krylon "Instant Mirror" paint. When absolutely dry, I covered that with a coat of flat black primer and reinstalled the prism.

It won't be perfect, but it should take care of about 90% of the problem. Otherwise, you'll need to get a junk body with a good prism and do a transplant. Be sure to clean the foam off of the donor prism carefully and don't replace it. The plastic hood should keep it in place fine.

Kino said:

I removed the prism of an Olympus OM1 that had pretty bad de-silvering, cleaned off the rotting silver with a qtip, masked off the clear surfaces with painter's tape and sprayed the entire prism with 3 light coats of Krylon "Instant Mirror" paint. When absolutely dry, I covered that with a coat of flat black primer and reinstalled the prism.

It won't be perfect, but it should take care of about 90% of the problem. Otherwise, you'll need to get a junk body with a good prism and do a transplant. Be sure to clean the foam off of the donor prism carefully and don't replace it. The plastic hood should keep it in place fine.

Click to expand...

Hey, sorry for reviving this old thread. I have a Nikon FM2 dug out from my uncle's collection that hasn't been used in decades and has pretty bad desilvering. I'm looking to try your method and wanna ask some questions. Did you use any specific solutions to clean off the rotting silver? And I assume any chrome mirror spray paint would be good for this purpose?

Even donor bodies are kinda hard to find around here and are pretty expensive if available, so I'm really keen to try this method.