3D model preparation 3D printing.

I have a 3D model that needs to be properly prepared and converted to .stl and .gcode files for further 3D printing.

I have various formats for the original file (Blender, 3dm, max, Maya, etc.). However, when I convert it to .stl and slice it for .gcode, some parts of the model magically disappear.

I am looking some one experienced in 3D modelling and 3D printing to properly convert (fix) it and create a proper .stl file for me.