I printed this little magnet/sign and I ended up having voids on the border and a cake icing texture on some of the letters. Other than on those letters, the white is completely smooth. It was printed at .16mm, ironing was 60mm/s, flow was 30%, ironing spacing .15mm, and ironing inset was .11mm. Bed was cleaned before printing, filament dry, etc. 
Are you able to share the 3mf?
How many top (and bottom) layer surfaces did you set?
Is the height of your model divisible by 0.16mm without a remainder?
The file is here: Darth Vader choking hazard sign - Free 3D Print Model - MakerWorld
Iām using a .4mm nozzle. Top layer is 4, bottom 3. The calculator gave me a whole number, so I assume it is divisible without a remainder.