3D Printing Thickness and Thin Faces Problem I am new to 3D printing and the printer in mind requires the mesh to have a minimum 2mm thickness. 



I started off by creating the shape with just a plane and extruding it out by 2.5mm. However the Print3D tool still says I have many "thin faces" (as shown by the selected faces in the reference image). I've tried recalculating the normal but the issue still persists. I've also checked that the mesh's scale in object mode is 1 so it probably shouldn't be an unit problem. 

Am I doing something fundamentally wrong regarding how "thickness" works when preparing for 3D printing?
Thin faces are not that the object is thin. A thin face is that you have two faces that are occupying almost exactly the same space. Somewhere you have created  for instance  a  face between four vertices  and then created a face  between three  of those  faces. The two faces occupy the same  space.
The distance between faces can be set in the 3d printing tool.