I think I found a solution! [:)]
A few days ago PtiNounours posted this message. Having finally been able to install SketchUp, I decided to install the plug-in he mentioned: FaceFront. After activating the plug-in, you "paint" over (parts of) your model and it reorients the faces of the model. In SketchUp you can't see a difference, but there definitely is a difference in the DAE-file when it's imported in SH3D. I treated two paintings this way, then did at least 5 renders, and each one came out correct.
Hans
