Every face that was printed against support material is visibly worse than the faces that were not: pitted, ridged, sometimes with fragments of support still embedded in it.
This one has a floor, and it is worth knowing where it is
Most faults on this site can be fixed. This one can only be improved, and being clear about that saves an evening.
An unsupported face is printed onto a surface built specifically to be weak and separable. The material below it is intermittent, cooler than a solid layer and deliberately not bonded. The plastic laid onto it is bridging over the gaps between support lines, sagging fractionally into each one, and then being pulled away from what it did stick to.
Nothing makes that equal to a face printed onto solid material or into air. Aim to make it acceptable, and design so the compromised face is not the one people look at.
What actually raises the ceiling
Enable a dense support interface. A separate layer of closely spaced lines between the support body and the part, which is the single biggest improvement available. Without it, the surface is bridging over the full support line spacing; with it, the spans are a fraction of that.
Reduce interface line spacing before increasing interface density elsewhere. Short spans sag less.
Set the interface Z gap to one layer height, not two. This is the direct tension in support settings: a small gap gives a better surface and risks fusing, while a large gap separates easily and gives a worse one. If parts are also welding to their supports, you cannot have both at once by tuning alone.
Keep cooling on for the interface layers. Plastic bridging a support gap behaves like any other bridge and benefits from setting quickly.
Slow the first layer over supports. Many slicers expose this as a separate speed; it is worth using.
The two ways to actually escape the ceiling
Different materials. Support in a material that will not bond to the part — PLA against PETG, in either direction — gives a surface far better than any air gap, because the interface can be dense and touching without welding. On a multi-material machine this is the answer.
Soluble support. PVA or BVOH dissolved away leaves a face with no mechanical removal damage at all. It is the best result available and it is expensive, slow and fussy about moisture.
Both are real solutions rather than mitigations, and both need hardware most people do not have — which is why the practical answer for most prints is the section below.
Orientation is the free version
Every model has faces that will be seen and faces that will not. If the part is rotated so the supported faces are the underside of a bracket, the back of a panel or a mating surface that gets hidden in assembly, the defect becomes irrelevant without a single setting change.
This is worth thinking about before slicing rather than after: rotating a part to move the supported face costs nothing, while cleaning up a scarred visible surface costs an evening with a knife and abrasive paper.
Cleaning up what you have
- Remove supports warm where the material allows. Warm plastic releases; cold plastic tears.
- Cut the interface, do not pull the tower. Flush cutters at the join give a far cleaner result.
- Scrape rather than sand for the first pass. A sharp blade takes the ridges off flat, and abrasive paper afterwards blends what is left.
- Wet-sand PLA and PETG if the finish matters; it clogs the paper less and produces a smoother result.
- Fill and prime if the part is to be painted. A supported face is a good candidate for filler primer, which is faster than sanding it flat.
Materials that leave the worst marks
PETG, because it welds to its own supports enthusiastically and tears when they come off. The polyamides, for the same reason and more so. TPU, which deforms during removal rather than releasing.
PLA is the most forgiving here and, combined with a dense interface, will give a supported face that only needs a light scrape. Fibre-filled grades break away unusually cleanly, since the filler makes both part and support less ductile — one of the few places where an abrasive filament is easier to work with than a plain one.