My sons have a very badly stained Primary mirror in thier 8" reflector that has not been re-aluminised in about 18 years! and it still gives fantastic views!!!
Seeing as I thought it would need re-aluminising anyway I scrubbed the daylights out of it to try remove the oxidisation and the stains but they didn't budge, and even after the brutal scrubbing I gave it the mirror STILL gives fantastic crisp images!!!!!!!! and . . . it was never over-coated!
Just bare Aluminium.
Don't ask me why this is so, Bert aluminised it, ask him why.
I am not suggesting you scrub your mirror
