Hi Michael, it's not the closed primary rail that is the problem, it is the sharp corners in the scaling rail.

Right now a scaling rail that has sharp corners in it will not work properly. Sometimes it may produce a kind of strange lump in the surface as in your first example, or other times it may fail to produce a proper surface at all.

For version 1.0 you'll need to use a scaling rail that is smooth through out, no sharp corners. In the future I can fix this up, it will involve splitting the sweep up into multiple segments between each sharp corner in the scaling rail.

- Michael