The deflectors are vital at warp speed but they’re still on at sublight speed. I also cannot find a single mention of polarized hull ever being used to repel dust/dirt at any speed. That’s why I asked. Memory Alpha doesn’t address it at all and google is returning zero results. I also cannot think of any recent dialog ever mentioning something to that effect. The closest I can think of is the Cerritos but their hull wasn’t polarized, it was had magnetic shielding.