Seams and zits on walls: where the seam comes from and how to make it less visible

How to recognise it

Every perimeter of a layer has to start and end somewhere, and that point leaves a small mark. Layer after layer, these marks form a vertical line on the wall (the seam, or Z seam) or, if the point changes on every layer, a scattering of dots called zits. Prusa and Bambu Lab point out that in FDM printing the seam cannot be eliminated completely: it can only be hidden or made less visible.

The seam is most visible on smooth curved surfaces such as cylinders, where there are no corners to hide it in. Large or irregular blobs, on the other hand, usually indicate too much material at that point: they are not the normal seam but an extrusion or filament defect.

The most common causes

Seam position: with “Random” the slicer picks a different point on every layer. Bambu Lab and Prusa warn that this makes the seam less uniform but leaves small zits all over the surface; for Prusa it makes little sense on parts with corners, where “Nearest” or “Aligned” hide the seam better in the corners.

Too much material at the start and end of the perimeter: Prusa lists extrusion multiplier and Linear Advance calibration among the settings that affect the seam (in Bambu Studio the corresponding calibration is called Flow Dynamics). Flow that is too high or uncalibrated pressure advance leaves a small build-up where the perimeter closes.

Wet filament: steam in the nozzle pushes material out irregularly and can turn the seam into a row of blobs. To recognise it and dry it, see the guide on wet filament.

Checks to do, in order

After each change, print a small cylinder and a cube, changing one setting at a time.

1. Check that the material and printer profiles in the slicer match what is installed: the official profiles already have flow and pressure advance calibrated.
2. Choose the seam position for the part: “Aligned” or “Nearest” for parts with corners, “Back” or “Rear” to hide it on the back of an object on display, “Random” only on round parts, accepting a few zits.
3. If your slicer allows it, paint the seam position by hand (seam painting) on the least visible areas of the model.
4. On curved surfaces, enable the scarf seam: in Bambu Studio from version 1.9, in PrusaSlicer with “Scarf joint placement”. It lengthens the print slightly and is less effective on corners and overhangs.
5. Calibrate the flow (extrusion multiplier) and pressure advance (Linear Advance or Flow Dynamics) for the filament you use.
6. Check the seam gap: a positive value stops the perimeter just before its end to leave room for excess material; in Bambu Studio the default is 15% of the nozzle diameter.
7. If the blobs stay large or irregular, dry the filament and try again.

Notes from manufacturer documentation

Prusa (PrusaSlicer): the position is set in Print settings → Layers and perimeters → Seam position; since PrusaSlicer 2.3 there is also a tool to paint the seam. “Random” makes the seam less visible but leaves small zits and, according to Prusa, increases the strength of the part; “Rear” moves it towards the back of the bed. “Staggered inner seams” offsets the inner seams from the outer one to make the part stronger. The scarf seam overlaps the start and end of the perimeter on smooth surfaces, lengthens print time and is less effective on corners and overhangs. Spiral vase mode avoids the seam, but with a single perimeter and no infill.

Bambu Lab (Bambu Studio): the available positions are Nearest, Aligned, Back and Random; “Seam gap” stops perimeter extrusion early (default 15% of the nozzle diameter) and the wipe speed is set as a percentage of the travel speed. The scarf seam, introduced in version 1.9, controls the nozzle height and the amount of material at the start and end of the perimeter; since 1.10 it can be enabled per filament, and the smart application uses it only where a normal seam cannot be hidden in a corner.

What users report in the forums

In a thread on the Bambu Lab forum, a user with an X1C saw a big blob at every seam when printing Bambu PLA: with random seams, round parts had actual spikes, while “Aligned” made the seam more visible. The problem was solved by drying the filament, after which the random seam was almost invisible. Another user, printing ASA and PA6, did not solve it by drying or by changing the seam gap. These are individual experiences, but they confirm that it is worth ruling out wet filament before touching other settings.

When replacing a part is justified

Almost never: the seam is managed with the slicer, calibration and dry filament. A dryer is useful if you often print materials that absorb moisture; replace the nozzle only if it is worn or damaged and makes the flow irregular.

How to verify the result

Record the model and variant, material, slicer profile and change made. Repeat the same test and compare the result with a photo taken before the adjustment. Keep the change only if it improves the defect without introducing others. Use your model’s instructions for temperatures, calibration and disassembly; this guide is not based on our own hands-on tests.