3D Print Blobs and Zits: Seam, Moisture, or Pressure?

Small raised dots are spoiling an otherwise good print. On one part they form a neat vertical line, like stitching. On another they appear at random, with fresh stringing. On a third they gather at corners and where the nozzle lands after travel. All three are called blobs and zits, but they are three different causes wearing the same bump. In the FixMyPrint community dataset, blobs and zits were the fourth most reported 3D printing problem: 1,320 of more than 20,000 help posts, with a 19.7% confirmed-fix rate. Those self-reported figures show how common the problem is — not a promise that one fix works 19.7% of the time.

A note on how this guide was written: compiled from published sources — Prusa's Knowledge Base (seam position, drying, stringing, Linear Advance), Bambu Lab's drying guidance, the FixMyPrint dataset, and clearly labelled community advice. We did not personally test these steps on a printer. Manufacturer-specific numbers are labelled; where no defensible universal number exists, the guide gives a test instead of a fake-precise setting.

Applies to: any FDM printer · PLA, PETG, TPU and the thirstier materials named in Branch B · PrusaSlicer, Bambu Studio, OrcaSlicer or Cura (setting names vary slightly). Evidence: Prusa's Knowledge Base, Bambu Lab's published guidance and the FixMyPrint dataset, plus clearly labelled community advice — full sources at the end.

In this guide: Which kind of blob is it? · The 3-question diagnosis · Branch A: Seam placement · Branch B: Wet filament · Branch C: Extrusion pressure · The settings worth writing down · FAQ

Key Takeaways

• The pattern is the diagnosis. A line at the layer starts means seam. Random bumps with popping, crackling or hissing mean wet filament. Bumps at corners and after travel mean pressure.
• Do not tune around wet filament. Moisture makes extrusion erratic and contaminates every retraction or pressure test that follows.
• A seam can be hidden and shrunk, not wished away. Every loop starts and ends somewhere; placement and pressure decide how visible it is.
• Three levers, three jobs. Flow = total volume. Retraction = travel and layer changes. Pressure Advance / Linear Advance = extrusion during speed changes.
• Every fix gets a verification. Change one thing, reprint the same test, and check the sign named at the end of that fix.

Step 0: Which Kind of Blob Is It?

Read the pattern before opening the slicer. Where a bump sits — and what else came with it — is worth more than any setting:

What you see	What it most likely is	Where to start
Bumps in a vertical line, at the same place layer after layer	Z seam placement — every layer's start/stop point stacked in one place	Branch A
Random bumps and pits, fresh stringing, and popping, crackling or hissing at the nozzle	Wet filament — absorbed water flashing to vapour in the melt	Branch B, before any tuning
Bulges at sharp corners, blobs at perimeter starts, or a lump where the nozzle lands after travel	Extrusion pressure — pressure advance, retraction or flow out of balance	Branch C
Every wall uniformly fat and crowded, on all sides	Over-extrusion as a total-volume problem	Branch C, flow first
Domes or sagging patches only on top faces, following the infill grid	Pillowing — a top-skin problem, not blobs	Our pillowing guide
Thin, gappy, weak lines everywhere — missing material, not extra	Under-extrusion or a partial clog	Extrusion and nozzle checks

Two quick confirmations. Compare the bumps with the slicer preview's seam positions: coincidence, layer after layer, is strong evidence for row 1. Then purge a little filament: a purge that bubbles or pops is material evidence and outranks slicer theories. (A clicking or grinding extruder gear is a different sound: a feed problem, not the moisture sign.)

The 3-Question Diagnosis

Answer in order. If more than one answer is yes, work the branches B → A → C: dry material first (wet filament makes later tests unreadable), seam placement second (a five-minute check), pressure calibration last.

Question 1: Do the bumps follow a map? A vertical line, the same corner on every layer, or the preview's seam positions. If yes — Branch A.

Question 2: Does the nozzle talk? Popping, crackling, hissing or sizzling, especially with fresh stringing, pits, or bubbles in the purge. If yes — Branch B — even if a seam line is also visible: tuning against wet filament teaches you numbers that expire when the spool dries.

Question 3: Do the bumps follow an event? They appear where motion changes — corners, perimeter starts, landings after travel. If yes — Branch C.

One discipline for all three branches: same test print, same filament, same temperatures, one setting at a time. A smaller version of the failed part is fine; a different model restarts the diagnosis.

Why It Happens (30 Seconds of Physics)

A blob is plastic that arrived at the wrong moment, and there are three clocks it can miss. The layer clock: each outer loop must start and stop, and any pause, ooze or pressure release at the join leaves a dot — stack the joins and the dots become a seam. The spool clock: absorbed water vaporises in the hot nozzle; Bambu Lab describes the melt expanding and extruding erratically, and Prusa lists blobs and bubbling among the signs of humidity-affected filament. Steam ignores retraction settings. The motion clock: the melt zone is a pressurised system, so when the head accelerates, decelerates, stops or restarts, pressure lags behind motion — exactly what corners, loop starts and travel landings expose.

Branch A: Seam Placement — the Bumps Are Where the Layers Start

Signs: bumps lined up vertically or on the same corner; the rest of the wall smooth. Prusa's seam position guide names this mark exactly — the start/end point of each perimeter loop — and warns it is most obvious on cylinders, which have no corner to hide in. This branch moves and shrinks the seam; it does not repeal it.

Fixes, in order:

1. Put the seam where the geometry hides it. With sharp corners, use Nearest or Aligned — Prusa notes Nearest can make the seam effectively invisible by tucking it into a corner. With a fixed display front, use Rear or paint the seam into a hidden strip. How you know it worked: the visible face no longer carries the line; the seam sits where you sent it.
2. On round parts, choose your trade consciously. A cylinder offers one deliberate line (Aligned or Rear) or Random, which Prusa describes as scattering the joins so the seam is less noticeable at the cost of a less smooth surface. How you know it worked: the continuous ridge is gone and the scattered marks replacing it are acceptable for that part — if not, return to one line in the least-seen place.
3. Turn on retract on layer change and wipe while retracting. Prusa's stringing and oozing guide recommends leaving both on: retraction removes the blob at each layer start, and the wipe drags the nozzle along the finished wall as it retracts. How you know it worked: the seam dots shrink without a matching dent or gap at each layer start.
4. On smooth curves, try a scarf seam if your slicer offers one. Scarf seams overlap the loop's start and end, hiding the join on smooth perimeters. Prusa notes the costs: more print time, and poor results on sharp corners and overhangs. How you know it worked: the curved test wall runs smooth under a fingertip at the join — no ridge, no groove.
5. Only now adjust the seam gap — one small step. Prusa defines it as the distance between the loop's endpoints: positive shortens the loop to reduce the seam; negative overlaps the endpoints and worsens a bump. Move a small amount positive. How you know it worked: the mark is smaller and the loop still closes — no groove or split. If a gap appears, step back.

If the line is still obvious on a smooth cylinder, that is the honest end of placement: the remaining choices are orientation, a painted seam, or finishing after the print. Bumps that sit exactly on the seam and nowhere else are not a pressure fault.

Branch B: Wet Filament — the Bumps Arrived With the Weather

Signs: bumps and pits scattered without a pattern; stringing that appeared on a profile that used to print cleanly; popping, crackling or hissing; bubbles in a purge. Prusa's drying guide lists poor surface quality as the general first signal of humidity, with blobs and bubbling in stronger cases; Bambu Lab's drying recommendations describe moisture vaporising in the nozzle and producing erratic extrusion, bubbles, stringing and rough surfaces. Community advice agrees on the sound: popping and fizzing are classic damp-filament signs in Prusa's forum. Polyamide (nylon), PVA and TPU need drying far more often than PLA, with PETG between.

Fixes, in order:

1. Run the control test: a known-dry spool. Load the same material with a storage history you trust and print the start of the same test with no setting changes. How you know it worked: if the popping stops and the surface cleans up, the original spool stands convicted — dry it (next fix). If the control spool behaves identically, moisture is not proved; go to Branch C and the hardware notes.
2. Dry the convicted spool actively, at its maker's numbers. Use a filament dryer or your printer maker's approved method, at the temperature and time in the table below (Prusa's values for Prusament) or your filament maker's equivalent. Respect the ceiling — Prusa warns that exceeding it softens filament until loops stick together. How you know it worked: a fresh purge runs quiet and smooth, with no bubbles or foam, and the retest's random bumps and stringing fall away — while a true seam line, if you also have one, stays exactly where it was.
3. Keep it dry while it prints and while it waits. Store the spool sealed with desiccant; for sensitive materials, feed it from a dry box. Prusa is explicit that its dry box slows absorption during printing and does not dry filament that is already moist. How you know it worked: the next print starts with the same quiet purge and clean surface as the post-drying test, not symptoms creeping back.
4. For the thirsty materials, make drying routine, not a rescue. Prusa recommends that highly hygroscopic materials — polyamide, polypropylene, PVA, BVOH and relatives — are kept dry or dried before every print; TPU is also often in need. How you know it worked: the improvement survives the whole test print — the last feature is as clean as the first.

If the spool was stored sealed and dry and shows none of these signs, do not force this branch. A day spent drying a dry spool is a day the real cause goes unfound.

Branch C: Extrusion Pressure — the Bumps Follow the Motion

Signs: bulges on sharp corners, a blob where a perimeter starts or the nozzle lands after travel, ooze between features — while long steady runs look right, and the bumps do not respect the seam alone.

Fixes, in order:

1. Re-establish the baseline: the maker's stock profile. Slice the same test with untouched stock flow, retraction and pressure values. How you know it worked: if stock prints clean, the cause lives in your changes — reintroduce them one at a time until the bumps return. If stock prints the same bumps, the profile is cleared.
2. Calibrate total flow — only if the excess is everywhere. Flow sets how much plastic every move lays down: right for uniformly fat walls, wrong for a corner-only bump. Follow your printer maker's extrusion multiplier calibration rather than guessing a percentage — Prusa's seam guide points to exactly this calibration. How you know it worked: the calibration check comes out on target, test walls are no longer uniformly oversized, and no gaps or thin lines appear in exchange.
3. Calibrate Pressure Advance / Linear Advance with the maker's own pattern. Prusa's Linear Advance guide describes it predicting pressure build-up and reducing extrusion just before deceleration — what prevents blobs at corners. Read the calibration print the maker's way: corners with blobs, value up; missing filament, value down. How you know it worked: the calibration line stays most even through its speed changes, and the test corners come out square — no bulbs, no notch or starved start.
4. Tune retraction as a travel control — one variable, small steps. Start from stock. If landings after travel carry the blob, adjust length or speed, not both, in a small step — and respect your machine's guardrails: Prusa caps retraction length at 2 mm on its MK2.5/S and MK3/S/+, while its Bowden MINI's preset is 3.2 mm. How you know it worked: on a travel-heavy test, landing points are clean and every restart stays full — no pits or gaps. Restart gaps mean you pulled too far.
5. Set extra prime (restart extra) to zero unless a gap sent you there. Extra prime pushes extra plastic out after a retraction to fill a short start; on a profile whose problem is a restart bump, it is a prime suspect. How you know it worked: the bump shrinks and no start gap opens. If one does, restore the smallest amount that closes it.
6. If ooze persists, lower the nozzle temperature in small steps. Hotter plastic is thinner and leaks more readily. Prusa's stringing guidance uses a decrease of 5–10°C as the test move; stay inside your filament maker's range and change nothing else. How you know it worked: less ooze and fewer landing blobs, with no new gaps or under-melted texture. If quality falls apart, restore the step.

The Settings Worth Writing Down

Two tables survive this guide's evidence bar: a qualitative placement table, and one manufacturer's published drying values for its own materials.

Part shape	Seam placement to try first	What it does	The trade
Sharp corners	Nearest or Aligned	Hides the join in a corner	Aligned leaves one deliberate, findable line
A fixed display face	Rear (or a painted seam)	Runs the line up the back	Only works if the part keeps its orientation
Smooth cylinder	Painted seam, or Random	Confines the join to a chosen strip, or scatters it	Random trades one line for many small zits
Smooth curves, scarf-capable slicer	Scarf seam	Overlaps the loop's start and end	More print time; weak on corners and overhangs

Material (Prusament)	Drying temperature	Drying time
PLA / rPLA	45°C	6 hours
PETG (all versions)	55°C	6 hours
TPU	60°C	4–6 hours
ASA	80°C	4 hours
PA11 Carbon Fiber	90°C	6 hours

Read the second table the way it was written. Those are Prusa's published drying values for Prusament materials. Bambu Lab publishes different parameters for its own filaments and methods, and your spool maker's label or data sheet outranks both. There is no universal retraction or Pressure Advance table here: those values belong to a printer, drive type, nozzle and material together, and the only published figures we could stand behind were single-model guardrails like Prusa's.

What the Community Data Actually Confirms

The FixMyPrint dataset (20,000+ community help posts, nearly 60,000 fix suggestions, classified by problem and by whether the asker later confirmed a fix) puts blobs and zits at 1,320 posts — #4 overall, with a confirmed-fix rate of 19.7%. For scale, #1 first-layer adhesion has 1,951 posts and 23.3%.

Read 19.7% as a floor, not a failure rate: people often fix a print and never return to confirm it — at least 19.7% provably solved, the rest unrecorded, not 80.3% unsolved. And the research provides no defensible split of successes between the three causes: anyone quoting "the seam causes X% of blobs" is inventing the X. Hence the branch order: pattern, sound, event — what you can observe.

What NOT to Do

• Do not call Random seam a fix for scattered zits. Random manufactures scattered zits on purpose. Already random? Placement is not your cause.
• Do not dry the spool and change three settings in the same afternoon. If the print improves, you have learned nothing.
• Do not keep increasing retraction until the blobs stop. Past your machine's working range the defect changes sides: restarts starve and the seam grows a dent where the bump was.
• Do not import Pressure Advance values from a forum post. Tuned on another printer, nozzle and material, the value is a random number with confidence.
• Do not sand first and diagnose later. Cleanup tells you nothing — the next print grows the same bumps in the same places.

When to Stop and Look at the Hardware

If a known-dry spool, a stock profile and all three branches leave the same bumps, stop tuning. Clean the outside of the nozzle first — Prusa notes that some filaments leave a film on the nozzle that strands stick to, and a dirty nozzle manufactures its own blobs. Then inspect for what a slicer cannot outvote: a partially blocked or visibly worn nozzle, filament dust or damage in the feed path, an extruder that clicks, grinds or chews filament. If extrusion stutters or stops, you have left blobs territory for a feed or clog fault. Bumps repeating at the same coordinates on one model only, across filaments and profiles, indict the model or its G-code — re-slice and read the preview.

FAQ

Q: Can the Z seam be eliminated completely?
A: Not in ordinary FDM printing — Prusa's position is that the seam will always be somewhat visible, because every loop must start and end. Vase mode avoids it only by restricting you to one perimeter and no infill.

Q: I switched the seam to Random and now the whole surface is pockmarked. Why?
A: That is what Random does — it takes the join Aligned stacks into one line and scatters one join per layer across the surface. On a cornered part, return to Nearest or Aligned; on a cylinder, compare one deliberate Rear line against the scatter.

Q: Can PLA really print wet? I thought only nylon needed drying.
A: Yes, though it suffers more slowly. Prusa notes PLA changes less with humidity than polyamide, but warns even PLA can absorb enough to cause issues — and its drying table includes PLA (45°C, 6 hours). The Branch B control test settles it for your spool.

Q: Should I always dry first, before touching settings?
A: Only when the moisture signs are present — popping or crackling, bubbles in the purge, stringing that arrived suddenly. A silent print with bumps exactly on the seam or on corners has a different cause; drying first teaches nothing.

Q: Why do my corners blob when the straight walls are perfect?
A: That is the pressure signature. On a straight run the melt zone is steady; at a corner the head decelerates, turns and accelerates, and pressurised plastic must be managed through the change. That management is Pressure Advance / Linear Advance (Branch C, fix 3) — not flow, which would thin the good walls too.

Summary

Diagnose before you tune: Step 0 → three questions → Branch B if the nozzle pops or the purge bubbles (control spool, dry at the maker's numbers, store dry) → Branch A if the bumps stack on the seam (hide it, retract on layer change, wipe on, scarf on curves, seam gap last) → Branch C if the bumps follow corners and travel (stock profile, flow if excess is everywhere, Pressure Advance calibrated, retraction in small steps, extra prime at zero, temperature down 5–10°C). Every fix ends with its verification — if the named sign did not move, that was not your cause.

Honesty note: This guide was compiled from manufacturer and community documentation — not personally tested on a printer in our workshop, and it does not pretend otherwise. Brand-specific figures and community findings are labelled as such; where the evidence ran out — a universal retraction table, a Pressure Advance value, a cause split — the guide says so instead of inventing a number.

Sources: FixMyPrint — dataset analysis (blobs and zits: 1,320 posts, #4, 19.7% confirmed-fix rate; self-reported); Prusa — Seam position; Prusa — Drying filament; Bambu Lab Wiki — Filament Drying Recommendations; Prusa — Stringing and oozing; Prusa — Linear Advance; Prusa — Prusa USS Drybox; community advice: Prusa forum — "Popping noise and bubbles".

Related guides: Stringing after changing PLA filament? Check these 5 things first · How to dry PLA filament: every manufacturer's numbers, compared · Why Is My Top Surface Rough and Puffy? Pillowing, Diagnosed in 3 Questions · First Layer Won't Stick on Your Ender 3? 8 Things to Check Before Touching Settings