I've tried to print the overhang test from this benchmark https://www.youmagine.com/designs/make-2015-3d-printer-shoot-out-test-geometries
The print went relatively well after some adjustments but I haven't been able to get rid of diagonal "scars" on the overhang. The scars consist of bits of the outermost shell that curled up. The shell is always printed counterclockwise, and the scars move clockwise along the shell. Adjusting temperature, cooling and speed doesn't notably change the number of scars, but reduces negative effects of the scars on the overall print quality.
This images show the best I've achieved so far:
Apart from the scars the quality looks very good on the 30 and 40 degree overhang and acceptable on the 60 degree overhang.
I'm printing ABS on an Ultimaker 2 (with added front cover and housing). Temperature is 240°C (I've tried 210-260), bed temp 100°C (tried 80-100), fan speed 40% (tried 0-60), speed 50mm/s (tried 20-100), layer height 0.1mm, 0.8 shell thickness and top/bottom thickness. The temperature in the build chamber is at about 50 to 60°C. I've also tried to print without front cover and housing.
Any thoughts on how to get rid of this scars and/or what might cause them are greatly appreciated.