The best way to place mullions on a curtain wall.
Curtain wall is made of an edit_poly object which the edges of editpoly represent the axis of mullions , the mullion object itself is a single line object with a sweep modifier on top. This approach allow you to have any type of cross section for the mullions while it keeps them always perpendicular to the surface, you can also select the edges of edit poly and force mesh-controller to place the mullion only on selected edges.
This is a simple technique to visualize the discreet curvature of mesh/poly/nurbs (All kind of objects) in 3ds MAX using Mesh controller in PARA . You also will learn how to use the custom attributes with parametric arrays.
Mesh controller provides access to Mesh data of geometries from all kind. With mesh controller you can work with faces/edges and vertices of the meshes. A short example of using this feature has shown in this video.