The wall looked fine in the preview and came off the plate covered in bumps. There is a raised pimple on every layer up one edge, a scatter of lumps across the face where nothing should be, and in the worst case a single growing lump that eventually drags the print off the bed. Blobs and zits are not one defect with one fix — they are four or five different mechanical causes that happen to look alike at arm's length. This guide reads the pattern first, works the fixes from free to paid, and ends where the honest answer is having the part made properly by a shop.
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Read the Pattern Before You Change a Setting
Pick the part up and look along the wall under a light rather than straight at it. Where the defects sit is more informative than what they look like.
| What you see |
What it actually is |
Section |
| One bump per layer, stacked in a vertical line |
Z seam — pressure released at perimeter close |
1 |
| Bumps scattered across the wall, no pattern |
Ooze deposited during travel moves |
2 |
| A bulge at the start of every perimeter, thin at the end |
Extrusion lag — pressure advance untuned |
3 |
| Every wall slightly proud, corners bulging outward |
Over-extrusion — flow uncalibrated |
4 |
| Craters, fuzz, matte patches, popping at the nozzle |
Wet filament |
5 |
| Dark specks and occasional smears, worsening over a job |
Carbonised residue on the nozzle |
6 |
| One lump that grows every layer until it hits the part |
Leaking hotend, not a slicer problem |
6 |
| Regular ripples spreading after corners, not lumps |
Ringing — a different defect entirely |
— |
That last row matters: ripples that fade out across a face after a sharp corner are mechanical vibration, and no amount of retraction tuning touches them. That is covered in why prints have ringing and ghosting.
1. The Seam Is One Zit Repeated Two Hundred Times
Every closed perimeter has to start and stop somewhere, and that junction stacks vertically into the Z seam. It becomes a bump because the melt chamber is still under pressure when the nozzle finishes the loop, so a small surplus escapes exactly where the loop closes.
Three settings do most of the work:
- Coasting or wipe. Stop pushing filament a fraction before the perimeter closes and let residual pressure finish the bead. Start around 0.04–0.1mm³ of coasting volume and raise it until the bump flattens — too much leaves a visible gap instead.
- Retract at layer change. A short retraction as the nozzle lifts stops it dribbling at the junction.
- Seam position. "Sharpest corner" hides the seam in geometry that already has a discontinuity. "Random" scatters it so no single line forms, at the cost of a slightly rough face. Most slicers also let you paint the seam onto a specific face — put it on the back.
A seam you can feel but not see is the realistic target. A seam that is genuinely invisible usually means the slicer has scarf-joined it, which not every machine supports.
2. Ooze Landing on the Wall During Travel
A nozzle full of molten plastic leaks whenever it is not printing. If it crosses open space and dribbles, you get stringing; if it crosses over the part and dribbles, or touches down still oozing, you get a blob welded to the surface.
Retraction is the primary control, and the right number depends on the extruder:
| Extruder type |
Typical retraction distance |
Typical speed |
| Direct drive |
0.5–1.5mm |
25–45mm/s |
| Bowden |
4–7mm |
25–45mm/s |
Raise it in 0.5mm steps on a retraction test tower and stop at the first value that is clean — more is not safer, and excessive retraction grinds the filament and pulls molten plastic back into the heatbreak, which causes clogs later. See why nozzles clog for where that ends up.
Then deal with the travel path itself. Enable "avoid crossing perimeters" so the head routes through infill rather than over finished walls, and turn on combing so it stays inside the part where possible. Z-hop is a trade: lifting the nozzle prevents it scraping a blob flat, but it adds a retraction-free pause at the top of the hop where the nozzle can drip. If Z-hop made things worse, that is why.
3. The Bulge at Every Perimeter Start
Look at a single perimeter closely: if it starts thick and finishes thin, the extruder is not building and releasing pressure in time with the print head's acceleration. That is extrusion lag, and it is what pressure advance — Klipper's name — or linear advance — Marlin's — exists to correct.
The calibration is a printed test pattern with a stepped value, and it takes fifteen minutes once. Typical landing zones are roughly 0.02–0.08 for a direct-drive extruder and 0.4–1.0 for Bowden, but the number belongs to your specific hotend and filament combination, not to the model of printer.
Slowing the whole print down hides this, because lower acceleration means less pressure to lag. That is a useful diagnostic and a bad permanent fix — you pay for it in hours on every job afterwards. Machines that ship with input shaping and a tuned pressure advance profile do this in firmware, which is a large part of why a current ELEGOO machine or a spare hotend assembly produces cleaner corners at speed than a five-year-old open-frame printer running stock firmware. Anycubic's US store is the other place to look at that end of the decision.
4. Flow: When Every Wall Carries Too Much Plastic
If the bumps are less "lumps" and more "everything is slightly proud, and the corners bulge outward", the extruder is delivering more material than the slicer asked for. Retraction settings will never fix this, because nothing is oozing — the wall is genuinely too fat.
Calibrate it as a measurement rather than a guess:
- Print a single-wall calibration cube with zero top and bottom layers, at a known extrusion width.
- Measure each of the four walls with stainless digital calipers and average them.
- New flow percentage = current flow × (target width ÷ measured width).
Two upstream things move that number. The first is the extruder's steps-per-millimetre — or rotation distance on Klipper — which should be verified before flow, by marking 120mm of filament and extruding 100mm. The second is the filament itself: diameter tolerance is a published spec, and a spool that swings ±0.05mm changes volumetric flow faster than any firmware correction can track. Brands that print a tolerance figure on the box, like Overture's PLA, are the ones worth calibrating against, because the calibration still holds on the next spool. If the whole wall is too thin instead, that is the opposite problem and it is covered in why prints under-extrude.
5. Wet Filament: Craters, Fuzz and Popping
Moisture absorbed into the spool flashes to steam at printing temperature and bursts out through the bead. The surface goes matte and rough, small craters open, and where the bead reforms behind the burst you get a raised lump. The audible tell is intermittent popping or hissing at the nozzle — if you can hear it, stop looking at slicer settings.
PETG, nylon, TPU and ASA absorb enough moisture to matter within hours in a humid room. PLA takes days, and a spool left open over a summer is not innocent. Drying is the test as well as the fix: run the spool at the manufacturer's recommended temperature for four to six hours and reprint the same file. A heated filament dryer box that can feed while it runs is the version of this that holds, because a spool dried on Monday is wet again by Thursday if it sits on an open rack. The same moisture mechanism is what makes filament snap on the spool weeks later.
6. When the Nozzle Is the Problem
Two distinct failures live here, and they behave differently.
Carbonised residue on the outside of the tip. Plastic that has crept onto the nozzle exterior cooks into a dark crust, then releases onto the part as a speck or a smear — typically worsening through a long job. Heat the nozzle to printing temperature and brush it with brass, and clear the inside with cold pulls using a nozzle cleaning kit. A silicone sock keeps it from coming back.
A damaged or worn orifice. A nozzle scraped across a print, gouged with a metal scraper, or run for 300–500 hours on carbon-fibre or glitter filament is no longer round, and the bead it lays is oversized and irregular whatever the flow number says. Cleaning does nothing for this. A nozzle assortment pack costs less than a failed print and changes in ten minutes.
And the one that is not a print-quality problem at all: a single lump that grows on the nozzle every layer, gathering plastic until it fouls the part. That is molten filament escaping between the nozzle and the heatbreak because the joint was not tightened hot. Heat the hotend to temperature, back the nozzle off slightly and re-tighten it against the heatbreak while hot. Left alone it becomes the "blob of death" that encases the heater block and needs the whole assembly replaced.
When the Surface Is the Deliverable
There is a point where chasing a seam stops being economic. If the part is client-facing, has to be painted, or is due tomorrow, an industrial machine with an enclosed chamber, tuned extrusion profiles and consistent material gets a clean surface first time — and SLA or SLS removes layer-start artefacts from the process entirely. The provider directory lists shops by location and process, and a single quote is usually cheaper than the third evening spent on a test tower.
Hero photograph by Kadir Celep via Unsplash.
