To answer the original question: watch this 3 minute tutorial https://www.youtube.com/watch?v=dkN8dN-FtGI - and then in Sketch Designer choose a Charcoal preset, turn off all the Background sliders, save as a custom preset. You're then good-to-go with sketching into outlines only.

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Learn the Secrets of Poser 11 and Line-art Filters.