Holes, Gaps or Bumps in the Top Surface

Likely causes, most common first

1. Too few top layers for the layer heightMost common
   Prusa suggests at least 3 top layers and notes thin layers need more: its example uses 9 top layers at 0.1 mm against 3 at 0.3 mm Starting point. Polymaker says too few layers leave the sagging first layer uncovered.
2. Infill too sparse under the topVery common
   Prusa explains top solid infill bridges over the infill, and lower infill means longer bridges and more sag. Polymaker calls raising infill usually the next most effective change for severe pillowing, after top thickness.
3. Infill pattern with long open gapsCommon
   Polymaker notes grid and rectilinear can leave long straight gaps under large tops, while gyroid and similar patterns support more evenly.
4. Weak bridging from material or coolingCommon
   Polymaker explains that ABS, ASA and some nylons print with little fan and so bridge poorly, needing more infill and top layers than PLA.
5. Under-extrusion or flow set lowOccasional
   Starved lines cannot close the surface however many layers you add. The OrcaSlicer calibration guide places flow ratio calibration after temperature, max volumetric speed and pressure advance. See Under-Extrusion.
6. Holes around features rising through the topLess common
   OrcaSlicer's surface expansion setting exists to fill the gaps left where a feature rises up through a top surface.

Fix it, one change at a time

1. Add top layers. Raise the top layer count or the top shell thickness. Polymaker suggests at least 4 to 6 times the layer height of top thickness Rule of thumb, for example 5 to 6 layers at 0.2 mm. In OrcaSlicer, the shell thickness setting adds layers automatically when the layer count alone would be too thin.
2. Raise infill density. Polymaker suggests roughly 15 to 30% for normal parts Rule of thumb, and more for very wide flat tops. Prusa offers a modifier that raises infill only in the layers just under the top, which saves material on big parts.
3. Switch infill pattern to gyroid or another pattern without long straight gaps, and preview again.
4. Check cooling for the top layers. For PLA, make sure the part fan is running. For low-fan materials, slow the top layers and rely on more infill and more top layers.
5. Calibrate flow if the lines are thin everywhere, following the calibration order in Flow Calibration.
6. For holes around raised features, use surface expansion if your slicer has it (OrcaSlicer releases after 2.4.2).
7. Change one thing at a time and keep the rest fixed, so you know which change worked.

Confirm it is fixed

Print a flat test plate the size of your real top surface, as Polymaker suggests, first with your current settings and then with the changes. The top should be continuous, with no pits or infill showing when viewed under a raking light. Then use the same settings on the real part.

Why it happens

Infill is mostly air. When the slicer starts the solid top, the first top layer has to bridge from one infill line to the next, and over each gap the line sags a little. The second layer lands on a slightly uneven surface and partly fills it in, the third smooths it further, and so on. Pillowing is what you see when there are not enough layers to cover the sag of the first one, or when the gaps are so wide that it sagged too far to recover.

That gives three levers, in order of effect: more top layers to cover the sag, more or better-distributed infill to shorten the gaps, and better cooling so each bridged line sags less. Layer height matters because the count of top layers is what covers the sag; Prusa's advice that thin layers need more top layers is about keeping the total thickness up.

Thin, separated lines on every surface are a different problem: there is not enough plastic, whatever the layout. Fix flow first in that case. For what each top and bottom setting does, see Top and Bottom Layers, and for the infill side, Infill Patterns and Density.