plastic fermenters here are at least 9 years old, taps have staining inside, no probs, hot water rinse and then left with bi-carb smeared onto inside, tap removed thread cleaned, stored up-side down on a towel, never pulled taps apart though they have been changed when the seal goes
before using - hot water rinse, sanitizer, hot water rinse left to cool, only ever had one bug and that was open top ferment in a stainless keg anyway, never in plastic
seasonal thing has merit
high useage may be more relevant as opposed to just being old, i don't stir in mine as i aerate by pouring so yeah, btw never seen a proper plastic fermenter break down, maybe that would be from harsh chemicals used to clean it
sour - would you say astringent, like sucking on a tea bag?