Thoughts on curl up/curl down on steep overhangs, cause and fixes.

I have spent a couple of days tuning printing with PETG on a couple of models with steep overhangs to investigate the causes and posible fixes. Heres my thought and I would love to hear your advice on the subject as well:

Picture of the topic(not mine)

https://i.postimg.cc/cJHw5mRz/curlup.png

1. Curl up:
   Means outerperimiter cools too fast compared to inner perimiter and infill. This causes the outer perimiter to contract faster than the inner perimiter, which again causes the outer perimiter climb up above or down below the inner perimiter. Another couse is the temperature-difference between current layer and the layer below, when the filament exits the nozzle. The difference will cause the whole layer to warp. This is not really what is discussed here.
   Fix1: turn down cooling fan, because this tends to affect outerlayers more than innner. This should minimize the temperature gradient between the outer and inner perimiter. Fix2: keep fan on, Turn up 'min layer time' if this is a small layer sub-20-seconds. this gives inner layers time to contract before external perimiter is applied.This should also minimize the temperature gradient between the outer and inner perimiter. Fix3: Improver overall cooling of the whole printbed with external fan to speed up cooldown of inner perimiter/infiil before outer perimiter is layed down.
   Fix4: tick the "external perimiter first" To laydown and start the cooling of this perimiter before the inner perimiters is layed down. This can work, but on very steep overhangs this runs the risk of 'curl down' instead.

2. Curl down:
   This can have the same cause as above, but aditionally this is caused by gravity, the viscosity of the plastic and the amount of contact it allready has with the inner perimiters and/or layer below.
   Fix1: Smaller layer-height to ensure layers dont get printed into thin air.
   Fix2: Improve fan cooling to ensure that plastic leaving the nozzle gets cooled fast which lowers its viscosity and its tendency to "drip"/curl down.

I hope my experience can help someone out there to get a better print quality. And please if anyone have something to add that I have missed, please do that.

Happy Printing