How to Fix Ugly Top Surfaces on Bambu Lab Printers

Pillowing (a bumpy, quilted top), pinholes that never close, or visible infill shadows through the top skin — top-surface defects all trace back to the same thing: the top layers are a roof, and the roof needs enough material and enough support underneath.

Why top surfaces fail

The top of a print is built by stretching solid layers across the sparse infill below. The first of those layers is basically bridging over the infill gaps; each additional layer smooths over the one before. Too few top layers, or infill too sparse to land on, and the roof never closes — you get sagging pockets (pillowing) and holes. Wet filament makes it worse, because steam pops leave craters exactly where the surface should seal.

Quick fixes

Advanced tuning

• Dry the filament. A cratered, rough top on a spool that used to print clean is classic moisture damage — PETG, PA and PVA are the worst offenders. Drying targets per material and brand are in the filament drying guide.
• Check the flow ratio. Slight under-extrusion leaves gaps between top-skin lines that look like scratches; slight over-extrusion leaves ridges the nozzle drags through. If you print non-Bambu filament, run the flow rate calibration in the Calibration tab — stock Bambu profiles sit between 0.92 and 1.00.
• Consider top surface pattern and speed only after the above. Monotonic line direction (a Bambu Studio default) already gives a uniform sheen; the big wins are layers, infill, moisture and flow.

Do the math on your roof

Top layers are a thickness, not just a count. On the default 0.20 mm layer height, 5 top layers make a 1 mm roof; switch to a 0.08 mm fine profile and those same 5 layers are only 0.4 mm of plastic bridging the same infill gaps — which is why a model that topped out fine at standard quality can pillow at fine quality. When you change layer height, think in millimetres of roof: around 1 mm total is a solid target, so thinner layers need proportionally more of them.

The same logic applies downward: the first top layer can only bridge the gap the infill leaves. At 15 % infill and a default grid, those gaps are several millimetres wide — long unsupported mini-bridges. Denser infill shortens them, which is why the 5-point density bump above works even though it seems unrelated to the surface.

Material notes

• PLA: irons beautifully — if the top matters, ironing plus 6 top layers is a near-guaranteed mirror finish.
• PETG: avoid heavy ironing; PETG loves to stick to the nozzle and drag. Fix moisture and flow first, keep ironing as the last resort.
• ABS/ASA: reduced part cooling means top skins stay soft longer; if tops look melted, add a top layer rather than more fan (more fan risks warping — see the warping guide).
• Generic/no-name spools: an uncalibrated flow ratio shows up on the top surface before anywhere else, because the skin has nowhere to hide excess or missing plastic. Calibrate flow first, then judge the top.
• Shiny-vs-matte patches on an otherwise flat top usually track print speed changes across the surface, not a defect — ironing evens them out if it bothers you.