Years ago I worked as an apprentice in a smash repairer. Mostly I was wet, sanding primer coats and sanding filler flat. This was 20 years ago, and I have not worked in this field since.
While doing this I never wore a mask, no one in the shop did that, except the painter when he was in the booth. No one online seems to wear a mask when doing these tasks either. I know the dust from this process probably is not fantastic, but I had no short term effects and was just hoping someone in the know can put my anxious mind at rest by confirming that the reason no one wears a mask because the dust is relatively safe as far as long term health conditions go. It seems highly unlikely these were fiberglass based as automotive fillers seem to be plastic-based. I also might have been in the shop when sanding of paint happened or smelt the smell of spray paint after a job but never remember being in close proximity to anything like that.