3. BASE - I used Magic Wand ( W ) on the drawing layer to select white background area. Than selected inverse (Ctrl/Cmd + Shift + I) and used Select / Modify / Contract with 1px. Filled the selection with grey color on a new layer below the drawing( Multiply layer style ). Than zoomed in to 250% again and used tablet and hard brushed Eraser (E) and Brush (B) to polish up the edges of the grey selection.