i ask my (hs) students to NOT smear their work at all. we do shading exercises similar to yours (we practice layering in different directions, too) and i demonstrate how to get a range of values and smooth shading using the pencil tip alone. i demo with an "automatic" pencil to make the point that all that is required is time and persistence... i also emphasize directional line shading for depth/texture effects; if the shading is smeared, that is completely lost. my gripe with smearing is that, in my opinion, it makes the drawing look blurry, smudgy, uneven, and sloppy. the line quality is lost, the definition, the sharpness... it seems like the lazy way out and a quick way to ruin a drawing. i feel much safer with and more able to control my pencil lines. showing my students others' pencil drawings that dazzle them helps me get my point (no pun intended!) across, too. but some still complain about not being able to smear...
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