An .STL file from Thingiverse was imported into Sketchup, defects cleaned up, until no defects were reported either by CleanUp2 or Sketchup itself upon Saving.
Preview in 3DS showed a solid block, and in fact a short print attempt began with a solid bottom even though there were openings all through it.
Matter Control seems to preview a good print of this model (attached below)
The S3D Factory file is also attached.
Are there perhaps settings that will inhibit the incorrect printing of of horizontal layers?