[ Ref. Doug Bullock in 180422 MMD ]

The "log cabin" look and porosity that was talked about in the 180422
issue of MMD applies to some 3D printing methods, but not all.  The
common 3D printing technique (FDM) that is now available for printing
plastic parts at home that starts with round spooled filament is like
this.

The more expensive SLA and Objet processes, however, are more like
a solid part when finished, although the surface will be stepped like
a topographic map.  The size of the steps will be determined by the
increment size in the build process.  These parts are more structural
and solid than the filament generated parts that tend to have different
characteristics depending on the direction (like wood grain).  A good
overview can be found at

  https://en.wikipedia.org/wiki/3D_printing_processes 

For metal parts, the processes are very different and the results are
dependent on which method is used.  For the most expensive methods, the
final parts can have up to 90% of the characteristics of machined parts
from solid stock.  A simplified explanation for the three current metal
processes can be found at https://pinshape.com/blog/3d-printing-metal/ 
including videos.

The bottom line is that the porosity and surface finish of metal 3D
printed parts is very much dependent on the process chosen, so be sure
to know what process makes sense for your application before having
parts made.

Jack Breen