Gaps in the top surface, in the walls or between infill and perimeters

Symptom

The top surface has holes, separated lines or bulges (the infill shows through), gaps appear between perimeters or between the walls and the infill, or holes appear on sloped surfaces.

Likely causes, from most to least likely

1. Too few top layers: the top skin is too thin, especially with small layer heights.
2. Sparse infill: the first top-skin lines bridge over the infill and sag.
3. General under-extrusion (partially clogged nozzle, low flow or high speed).
4. Slicer options disabled: ensure vertical shell thickness (holes on sloped surfaces) or fill gaps between perimeters.
5. Not enough cooling while laying the first top layers over the infill.

Checks to tell which one it is

• In the slicer preview, count the top layers and work out the total top-skin thickness.
• Check the infill density: the lower it is, the longer the bridge and the bigger the sag.
• Look for signs of under-extrusion elsewhere on the part (thin walls, separated lines).
• Check the preview for gaps between perimeters or on sloped areas before printing.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Add top layers

   Increase the number of top layers to keep the top skin thick enough. As a Prusa guideline: 3 layers at 0.3 mm layer height and 9 at 0.1 mm.

   Check: The top skin is closed and the infill doesn't show through.

   If that does not fix it: Adjust the infill.

2. Adjust the infill

   If the top skin sags, raise the infill density a little. Prusa says most models are fine at 10-15%, but the less infill, the bigger the sag of the first top-skin lines.

   Check: The first top-skin layer is well supported and the surface is smooth.

   If that does not fix it: Fix under-extrusion.

3. Fix under-extrusion

   If lines are missing all over the part, check the nozzle, extruder, speed and extrusion multiplier before touching other settings.

   Check: Walls and top skin come out continuous.

   If that does not fix it: Enable the slicer's wall options.

4. Enable the wall options

   Enable ensure vertical shell thickness to avoid holes on sloped surfaces, and gap fill if spaces remain between perimeters.

   Check: The preview no longer shows gaps between perimeters or on sloped areas.

   If that does not fix it: Improve top-skin cooling.

5. Improve top-skin cooling

   Check that the part cooling fan works and is enabled in the profile (except for materials such as ABS, ASA or PC, which use very little of it).

   Check: The first top-skin lines don't droop over the infill.

   If that does not fix it: If it persists, compare the whole material profile with the manufacturer's.

Prevention

• When lowering the layer height, increase the number of top layers to keep the top-skin thickness.
• Check the slicer preview before printing important parts.

Still not fixed?

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Printers it applies to

Any FDM (filament) 3D printer, from any brand.

Official sources

This guide summarises the official documentation linked here:

• Layers and perimeters · Prusa Research
• Infill · Prusa Research
• Under-extrusion · Prusa Research
• Cooling · Prusa Research