Blobs and zits

Little pimples on the outside wall. They form where the nozzle starts or stops a perimeter — pressure inside the nozzle does not vanish the instant the extruder stops, so a bit extra squeezes out. Modern slicers can compensate for this well.

Looks like: Small bumps or pimples scattered on the outer wall, often lined up in a vertical seam.

Fixes, in the order worth trying

1 · Try this first

Enable pressure advance / linear advance

This is the real fix. It models nozzle pressure and adjusts the extruder ahead of time so flow starts and stops cleanly. Once tuned, it removes blobs at line ends and also sharpens corners.

Klipper: run a pressure-advance calibration tower and set the value per filament (typical: PLA 0.03–0.05, PETG 0.05–0.08 with a direct drive). Marlin: use Linear Advance with M900. OrcaSlicer has a built-in pressure advance calibration under Calibration.

2 · Try this first

Move or randomise the seam

Lined-up zits are the Z-seam — the point where each perimeter starts. You cannot remove it, but you can put it somewhere it does not matter, or scatter it so it never forms a visible stripe.

Z Seam Alignment: Cura → Shell → Sharpest Corner, or User Specified to hide it on a back face
Seam position: OrcaSlicer / PrusaSlicer → Process → Quality → Aligned with "rear" or Random

3 · Common cause

Turn on coasting and wipe

Coasting stops the extruder slightly before the end of the line and lets residual pressure finish it; wipe drags the nozzle over already-printed material to leave the excess where it will not show.

Enable Coasting: Cura → Experimental → On, volume 0.05–0.1 mm³
Wipe on loops: OrcaSlicer / PrusaSlicer → Process → Quality → Enabled

4 · Common cause

Lower the temperature

Cooler plastic oozes less between the stop and the next start. If blobs come with stringing, treat them as the same problem and see the stringing page.

Printing Temperature: Cura → Material → −5 °C per test

5 · Less common

Check retraction is not over-aggressive

Too much retraction can pull in a bubble of air that comes back out as a blob at the next start. If you added retraction to fight stringing and gained blobs, you overshot.

Retraction Distance: Cura → Travel → Direct drive: keep under 1.5 mm
Retract on layer change: OrcaSlicer → Filament → Setting Overrides → Try disabling if seam blobs are per-layer

Change one thing at a time. If you adjust temperature, retraction and speed together and the problem goes away, you have learned nothing and you will be back here next month. Re-print the same test object after each single change.

Frequently asked questions

What is the Z-seam and can I get rid of it entirely?

It is where each layer's outer perimeter begins and ends — an unavoidable consequence of printing a closed loop with a single nozzle. It cannot be eliminated, only hidden or minimised. "Scarf" or sloped seams in newer slicers come closest by blending the join across a distance.

Why do the blobs only appear on one side of my print?

That is the seam alignment doing its job — most default settings put the seam in a consistent place. Set seam position to "Random" to scatter it, or "User Specified" to point it at a face nobody sees.

Does pressure advance need re-tuning for every filament?

Ideally yes, per material type rather than per spool. PETG needs a noticeably different value from PLA because it is more viscous. Different brands of the same material are usually close enough to share a value.