Fixing a non-manifold mesh
Note: All images are linked to full size versions.
This tutorial is based on Blender 2.56a beta interface (see getting started section of the Blender site tutorial page). I will show you how to fix a non-manifold mesh problem. What is a non-manifold mesh ? Look up the word manifold first. Then watch this video.
I created a cylinder in Blender and then used a couple of boolean operations that are probably responsible for making the mesh non-manifold. After some discussion on #blender IRC channel I discovered the process to fix this.
A non-manifold mesh
Add a Decimate modifier to your mesh. If it is non-manifold then the modifier will give an error that you can see in the following screen shot.
Select the non-manifold parts of the mesh
Select the model. Enter Edit Mode and choose the menu option Select --> Non-manifold.
Apply Edge Split modifier
What now ? We could fix the mesh manually (see the section below ) but there is an easier way. Blender does not have a "cleanup" function as shown in the Maya video above but here we can just use the Edge Split modifier.
Make sure the non-manifold mesh edge is selected (as shown in previous step) and then add the Edge Split modifier. You might get a message about modifiers not having been applied. You can ignore that unless you have other modifiers set up to the ones shown, if so apply those first.
Set the the Edge Angle to 90 degrees (may vary if you're using another model to the one shown).
Apply the modifier and ... hey presto !
Behold a fixed mesh
Notice the Decimate modifier is now able to count the polygons in the model (take model out of Edit Mode first). The mesh is now manifold and should be in the right state to be unfolded or unwrapped using UV Unwrap (Mesh menu).
Potential for manually fixing the mesh
There may be times when you need to look at the nuts and bolts of the mesh. One way is to show all the normals for the faces (the direction that the shader faces are pointing in).
To do this use View --> Properties --> Normals --> Face
If there are non-manifold parts of the mesh then you would see inconsistencies in the face normals (a lack of a blue line, or pointing in the wrong direction). See the video at the top of the page for more hints for manual fixes.