OH MY GOD. people have recommended that one very handy fill in tool for clip studio but it doesn't work for me 99% of the time 
AND BEFORE ANYONE SAYS INCREASE THE CLOSE GAP THING I ALREADY TRIED
I concider myself to have pretty neat lineart but because I work quickly and focus more on brush stroke, I don't tend to close in some areas!! And a lot of the time I don't notice. So going back and fixing it is JUST UGH
If you look closely, you'll see there's a ton of gaps in some areas! (most people don't notice when reading because I pride myself on line confidence heheh) BUT I will never not feel the pain of using the magic wand tool and it selecting half the canvas 
Could be worse, but UGH. WHEN YOU HAVE NEAT LINEART YOUR COLOURING NEEDS TO LOOK CRISP AS WELL 
I dunno, please tell me someone understands my pain, or give me tips or smn