I succesfully filled a hairline plaster crack on my wall, so decided to tackle one on the professionally plastered/painted ceiling. I used fine surface filler, and smoothed it with a filler knife, but it wasn't completely flat. I used a sheet sander, which should have sanded flat but sadly has removed the existing paint in some areas. See pic. I probably need to refill them, don't I, rather than just paint over them? 😟