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I've seen a lot of old SCTs with perfect coatings on their primaries. The ones I haven't have either had the dust cap left off the back for a long time, or someone removed the corrector plate and had a fiddle with things inside (Why, I'm not sure). Usually its just the corrector that gets dirty. I don't think I've ever heard of an SCT primary having to get recoated, probably because they are quite well protected inside the tube.
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