Having problems with the paint on my E36 door mirrors

[MD_SANDERS]

Just wondered If anyone with a compact or any E36 for that matter has come across this problem.
After recently cleaning and polishing my car (which on the whole is in extremely good nick), I noticed behind the door mirrors, the black paint was bubbling and falling off.
As though something like salt had got up behind it and was reacting with it?
Has anyone else come across this problem, or is it just an excuse for me to change my mirrors to some nice M3 ones or the like?

Cheers

Matt