Two of the edges are sharp 90 degree edges for inner corners and crevices, while the other two edges are gently filleted around a concave inner slope for sanding rounded surfaces.

The .stl file itself is a solid object, but to get the honeycomb effect in the picture, print the part with:

• 0 top and bottom layers
• at least 5 perimeters

Supports are not needed, just print the part on it's side. 

Scale the model as large or as small as you need for your sanding requirements. Object defaults are 60mm L x 30mm H x 10mm or 20mm D