Mesh bisect with fill doesn't place triangles for the cut surface My task is to import a closed mesh into Blender, apply Bisect with the fill option, with the desired result being that the new body would also be a complete mesh. 

However, the result I get has the bisect face filled with "grey", but no mesh. And an STL exported from that is similarly missing triangles in that area.

Example: Starting STL, a cylindrical cup, imported into Blender:



After applying the bisect tool along a horizontal line (ie: a horizontal plane):

 

As can be seen, the face that was cut has no triangles. And if exported to STL, that area is missing, with the overall shape not being closed.

Have I missed some option in bisect that properly fills in the mesh? Is there some subsequent step that's needed?
The "Bisect" function did generate a surface at the cut area, though it's an N-gon, STL-export seems to not offer an option to triangulate N-gons automatically. 

Solution:

You can select the cut surface in Edit mode with Face selection and then use "Triangulate Faces" in the "Face" Menu Ctrl+F. Or use the direct shortcut, in this case Ctrl+T.

Here an example:



This should do the trick.