Originally shared by Marcus Wolschon
Who has actually printed in 20 micron layer height on an Ultimaker II and doesn’t work for Ultimaker?
What can you say about failed prints, calibration steps,… ?
Did you use a lower resolution raft to counter tiny inperfections in leveling?
The manual https://www.ultimaker.com/spree/uploads/36/original/Ultimaker_2_User_Manual_V1.08.pdf
calls for a leveling using a sheet of paper. That would be about 0.85-0.11mm thick and far thicker then 0.02mm.
Usually you’d set your first layer thickness to at least 0.2mm to compensate for a somewhat uneven bed surface. Getting it trammed/leveled/squared to 10 microns or better (which is the ballpark you’d need for a 20µm layer to stick) is absolutely impossible imo.
How does it work out after the first layer?
Anyting to watch out for except the extreme times?
I don’t have a Ultimaker 2 but would happily use it, if
@Ultimaker wanted to send me one. 
20 microns should be easy enough, though. The trick would be to set the 1st-layer height to a more reasonable number and let that act as a level (or print a raft).
This is similar to the approach I used to print < 1 micron layers on my Ultimaker 1. The hard part isn’t getting an Ultimaker to move at these types of resolution - it’s getting it happy enough to move really fast, which you need to keep the extruder moving, and waiting forever for a print to finish…
So you have to keep a minimum extruder speed in mind. Given that it uses 3mm filament that could be a problem indeed.
If you’re printing down under 0.025mm, your machine is probably in fairly decent shape.
Extruder speed isn’t really a huge problem unless you have layers with small area - if you have those, you’ll need to disable min-layer-time or balance it vs the min usable feed rate that keeps the extruder moving.
I don’t have it set. 5-8 weeks of waiting.
However I know that I’ll print a lot of important things as high resolution as possible to increase the level of details visible and reduce the sanding required for a perfect surface.
IMHO, 1.75mm filament is better tuned for this. With 3mm filament there’s so much volume of material that at 0.025 resolution, you’re basically printing with the ooze.