I draw my lineart with Paint Tool Sai. When it's finished, I save the file as .psd to continue my work in GIMP - simply because I love colouring and shading with that free-of-charge piece of gold
So in GIMP I duplicate the lineart layer as many times as necessary to create layers for each "coloured surface" of the picture (e.g. shirt, fur, trousers, hat and so on). This eliminates the danger of overflowing colours and all kinds of smudges (so I've been told). The problem is, duplicating the lineart layer more than a couple of times makes the inking look thick, blocky and ugly.
For example, compare my latest picture Dawg with its lineart which I uploaded a bit earlier (in full view). I'm doing something wrong - so please help me! How on Earth do you people avert this? How do you shade and how do you keep your lineart so nice and smooth?