Many thanks for your reply.
The most common thing I want to do is increase the number of perimeters, or the extrusion width, or both, for a part of a print to increase strength in regions of high stress without adding too much weight.
However, I find setting the top layer to 0 on the first process and bottom layer to 0 on the following process can result in incorrectly printed parts for some shapes.
I attach an image of a typical scenario where I would like to use multiple processes.
This wing section is prone to fracturing at the stress point where the flap cutout starts, towards the bottom right in the image. I would like to be able to reinforce the wing section for a few mm around that height by switching from 1 perimeter to 2.
Unfortunately, setting the top layers to 0 on the first process (1 perimeter process) results in the servo cutout not having a roof, while setting the top layers to zero on the second (2 perimeter process) results in the trailing edge having no roof where it extends beyond the flap cutout.
So I have to choose between having these openings or having heavy solid infill right across the profile of the wing at the process boundaries.
Currently the only way I can see of achieving what I need is to leave the top and bottom layers non-zero and write a program to parse the g-code output and remove the unwanted solid infill layers, which is a pain.
Using multiple process to vary the properties of a print as a function of height is a powerful feature of S3D, but without an option to suppress unwanted solid infill layers at process transitions, it is unusable for some applications.