Extra external layer (shell) on my first prints

Question

I got my first 3D printer this week, a Creality Ender 3.

Tried to print a few small samples to test it, but every shape I printed had an extra external layer. Please, check these images:

I used Freecad to draw the objects, and Slic3r to convert them to G-code.

Am I doing something wrong? Is my printer not a good one?

Any comment will be very helpful.

Answer 1

If you do some quick calculations it appears that this is caused by under-extrusion.

Comparing the extruded volume to the calculated volume for the deposition there is a mismatch of about 2.6. This hints to the use of an incorrect diameter for the filament $\frac{2.85^2}{1.75^2}=2.65$.