I've had a couple of prisms where the silvering has been damaged due to decomposed foam .
This left a big distracting black mark in the viewfinder .
The last one I cleaned up was an unmetered prism for a Bronica ETRSi , bought for next to nothing .
Where the silver was missing I used a silver , oil based marker pen to colour it in , rather than look at a black mark .
Without the black mark , it's perfectly useable , showing a full image of the focus screen .
The effected part just lacks a bit of brightness and contrast etc , but still perfectly useable .
Example of a suitable pen ;
ebay , silver pen link
I'm not sure how effective this would be on a 135 SLR as the prism is much smaller , on the MF prism , the damaged area was probably a smaller percentage of damage , but wort trying first .