Jackson's Art - The smoothest shading results can be most easily achieved by using Zest-it Pencil Blend. It is a solvent that will break down the oil and wax content of pencils,
Zest-It Pencil Blend Medium for Colour Pencils 125ml | eBay
ZEST IT PENCIL BLEND | RECOMMENDED DRAWING TOOLS
Zest-It Pencil Blend for Coloured Pencils, Wax Pencils & Oil Pastels 125 ml | eBay
Zest-It Pencil Blend 125ml | Artcoe
Blending Pencils with Zest-It Pencil Blend - Jackson's Art Blog