Greetings
TL;DR
It would be very nice to be able to change speed and disable cooling fan for the very first layer on top of support interface layers. Reason is to better tune adhesion properties. Essentially, we’re talking here about classic first layer bed adhesion issues, though the ‘bed’ in this case is a layer of PLA with PETG being laid on top.
Suggestion
Support -> Use first layer settings atop interface.
Background Explanation
I have enjoyed great success using PLA/PETG combinations for soluble solid support. That is, support structures using non-soluble PLA with zero interface gap when printing with PETG for the main part. The interface comes away very cleanly and easily, leaving near perfect print quality, compared to break-away support with their inherent separation gaps used to lower adhesion.
For best results when printing PETG atop PLA on my X1C speedster, I have found a need to slow the printing speed and at times even turn off the part cooling fan.
Currently, the only way I have found to slow the speed for this layer is to reduce internal solid infill speed to 40mm/s. This works well but of course it also unnecessarily slows solid infill layers above the first one. Not a deal breaker in itself but then there is the cooling fan issue.
Turning cooling off (to raise temperature) is not an option in the slicer that I can find. So, I’ve been doing that manually. In fact, this step tends not to be strictly required but can definitely help when there is a large X/Y area of PETG being laid down over PLA — otherwise, shrinkage at the edges can cause curling followed by print head collision and layer detachment.