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I'm sorry: what do you mean by look like a sketch? I sketch in charcoal, in conte crayon, in drybrush and water color -- and they all look different. I can generally get similar looks in Gimp by choosing similar brushes and drawing large enough. What I really like to do in gimp though is take the default brush or bigger, select my palette then just draw on top of everything so parts of the lines are covered up and obscured. Try it. You may like the results. Otherwise, just select or download the right brush size and draw big.