My 3 fermenters are over 10yrs old. For the first year trying on and off with the older style coopers fermenter I never had a good beer.
Got a new coopers fermenter a year or two later, still bad beer. Bought a 25L pail for a third fermenter a year after that, still bad beer. Half my brews were infected and foul so I gave up on brewing.
This year I started brewing again and the first few were also not very good brews on all three fermenters that are now 10+ yrs old. I bought new taps, grommets and lid O rings.
Still had a 50% chance of a bad brew.
Then using the same fermenters, beers started to become better, but I stopped brewing again due to high temps. While waiting for cooler temps one even got that pink mould on it. But that one and the two other 10+ year fermenters that have made some very bad and infected beer are now making good beer.
Change taps, o rings, grommets, air locks and sanatize well is all I needed to do. Scratches and stains haven't had any effect.
More chance of a beer going bad due to the bottles/bottling IMHO. The bad beers I made early this year were all in equally old plastic bottles. Once I switched back to glass bottles, most of my problems have gone away. In fact 10 years ago, most of my bad beers were in those PET bottles that gave me trouble this year. Those things are almost impossible to clean.
E: I have a habit of saving the trub for yeast, and even in the brews that ended up bad the sample saved for yeast was still good, so it didn't go bad for me in the fermenter most of the time, but rather the bottles/bottling. That is, these days that is the case. Back in the old dark days, things went bad in the fermenters, but the point is I'm still using those old fermenters now just fine and getting decent beer out of them.