The top surface has visible holes, thin stripes, or sections that didn’t fill in. Looking down at the top of the print, you can see infill pattern showing through, or islands of solid material separated by gaps.
Identify the Pattern of Gaps
Repeating pattern matching the infill below: Too few top layers. The solid top layers are bridging over infill air gaps, and not enough layers have built up to fully close the surface. This is the most common cause.
Random gaps scattered across the surface: Under-extrusion. Not enough material is being deposited. Check for partial clogs or flow rate issues.
Gaps only in one direction (parallel lines): Top surface speed is too fast, or the extrusion width is slightly under-set for the top layer pattern. Switch to monotonic or ironing fill.
Gaps that appear after a long print but not at the start: Wet filament progressively getting worse with heat, or a gradual partial clog building up from debris.
Fix Sequence
1. Add more top layers. The minimum for a clean, gap-free top is 5 layers at 0.2mm layer height (1.0mm total), or 7 layers at 0.15mm. Less than this and the top layers spend most of their depth bridging over infill rather than building a solid surface. Going from 3 to 6 top layers fixes most pillowing and gap problems immediately.
2. Increase infill density. Low infill (10–15%) means large gaps between infill strands, which makes top-layer bridging spans longer and more likely to sag. For a clean top surface, 20–25% infill gives the top layers enough support. Higher is not better — 40%+ adds print time without meaningful surface improvement.
3. Check flow rate. Print a single-wall vase mode cylinder and measure the wall thickness with calipers. If it’s thinner than your extrusion width setting, you’re under-extruding. Raise flow rate 2–3% at a time until walls measure correctly. Don’t raise it blindly by large amounts — over-extrusion creates a different set of problems.
4. Slow down top surface speed. Top layers print over bridged spans and need more time to settle flat. 20–30mm/s for top surfaces (not 60mm/s like infill). Most slicers have a separate “top surface speed” setting.
5. Switch fill pattern. Monotonic top fill ensures each line overlaps the previous one in the same direction, eliminating the gaps that occur with alternating rectilinear patterns. Enable it if your slicer offers it.
When It’s Actually Pillowing
If the top surface has small bumps or a puckered texture rather than gaps, see the Pillowing guide. Pillowing and gaps look similar but have different root causes and fixes.