Lemme Guess!
It's a GAF or a Chinon, right?
I've had a couple of GAF (Made by Chinon) ST-602s over the years and the worst thing about them is that the rubber eyecup deteriorates to tarry goo. To my knowledge this is unstoppable, but if it's the camera I'm thinking about the eyecup is not hard to replace, and I know several places to get substitutions for it.
I was using one of them a couple of weeks ago without knowing that this one too has the problem, when my wife aked me "What's that black mark on your face?".
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