New plaster line shows.

My answer simply line your walls and repaint

Hi, You will be able to rub it down over the line yes to make it less noticeable, But it wont be easily to see what its like until you have painted it again, using a good sander would be the best option, although a bit dusty. and if you are rubbing down the actual plaster to get it flat then yes you will need to apply a mist coat again to the bare plaster, then recoat twice with your top coats. Once you have done the watery mist coat you may be able to tell if its ok or not. As far as other options are concerned, It depends how high the line is, and is the line the same height all the way around, If you were lucky, and the line was at the correct height all the way around, maybe you could add a dado rail to hide it, Maybe you could add panelling to the bottom half to hide it. You also could reline your walls as somebody else said, but depending on the size of the bump this may still be noticeable through the lining paper. Without seeing it its hard to say really. Hope this helps

Seek for professional to assist the issue and give you the best appropriate solution.

I would lightly sand where the join is and apply a light skim of filler along the join. Once filler is dry sand then mist coat filler to seal it then repaint. I have done this many times over the years without any issues.