The smoothing plane is normally associated with the finish surface of a board, the last plane to hit the wood before the finishing stages.  Regardless of the form that you have, either bevel up or down, this tool can produce glass smooth surface on your work surface.In the Stanley numbering system, the smoothing plane would be either a number 4 or 4 and one half. I don't want to get too much in the technical details of the dimensions that this type of plain should have, many have already covered this information better then myself and is not the purpose of this post.

Form

I personally own 2 version of smoothing planes, a bevel up versionand a bevel down version, which would be called a No. 4 in the Stanley numbering system.Yes both are Lie-Nielsen but the brand is not really important here.  The main idea is the purpose of this particular tool and how to use it to the full potential of the tool.  Also, to me, although the bevel up version could be classified in the block planes, to me, it is still a smoothing plane, just that the iron is oriented differently.  Regardless, either plane serves the same purpose in the end.

Use

As mentioned, either plane is used to finish up the surface of a board to glass smooth.  Yes it is possible to finish to this level without the noise typically associated with power sanding.  Personally, depends on the wood I use.My first reflex is to go to the smoothing plane to finish and not the power sander, unless I really know the wood will not cooperate well.  Yes I do own a ROS for these exceptions.Most of the pieces that I will finish up with the plane are fairly small pieces :

• Edges of small boards like table rails

• Small surfaces like table rails or small panels

• Any other surface that are relatively small that needs finish surface

I do use both smoothing planes in slightly different ways, mainly setup for different tolerances :

•  Bevel down No.4

  • This plane is setup for probably a shaving of 1-3 thousands of an inch so I guess it is setup on the thicker side of things

  • I use this plane mainly right after a rapid pass of a jointer plane.  To my, the surface left by this setup is good enough for glue joints for smaller pieces, even longer one

  • pass just before the finish pass left by a jointer plane, just removing any bigger marks left by the previous plane

  • depending on the surface, this is sometime good enough, specially for non show surface.  Some might argue that it's too much, and in some cases, it is and just forgo this step

  • The sharpness of the iron is normally pretty high, not hesitating on going back to the sharpening stones to refresh the edge

  • Bevel up Smoothing

    • This is what I consider my finish plane, setup for high tolerance cravings, typically in the 1-2 thousands.

    • Got multiple irons for this plane, honed at different angles to attack different grain situation.  All kept razor sharp and they see the stones most often.

    • In order to get these shavings, the board has to be pretty flat to take continuous shavings.

I have the luxury of having 2 smoothing plane.  Could I be using just one ? Definitely, but I have the luxury to have a coarser tool in this category that I don't need to change settings to do the finest work.As for the choice of bevel up versus the bevel down debate, most if not all can be done with one or the other and not needing both version.  Options in the bevel down do exist to change the frog angle to change the angle of attack to help in more difficult woods.I believe that regardless of the number of a particular plane or options that you outfit the tool with, as with about any other plane, please keep the iron sharp and learn to keep it sharp.  Use what ever method that you feel comfortable with, stick with it and go learn how to sharpen if you have the chance to.