Bubbles and craters in varnish: causes and fixes
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A full-coverage gloss varnish is meant to enhance the motif. Instead, tiny bubbles, bright dots or small craters show up in backlight. The cause is rarely the varnish itself, and usually the substrate, dust or the settings. A few tests will narrow it down well.
How varnish works in general is covered in the article Varnish in UV printing.
First, decide: defect or intended texture?
Not every rough surface is a defect. In matt mode, UV varnish is deliberately cured immediately after it lands. The droplets cannot flow out and remain as a fine texture that scatters the light. In gloss mode, by contrast, the lamp cures a little later so that the varnish can level out. With relief and texture, too, the surface is meant to be uneven.
This is how you tell the two apart:
| Feature | Intended texture (matt, texture) | Defect (bubbles, craters) |
|---|---|---|
| Distribution | even across the whole varnished area | individual spots, clusters or only on certain materials |
| Shape | fine, regular grain | round bubbles, rings with a dip in the middle, fluff |
| Repeatability | the same in every print | changes from print to print or follows the material |
| Setting | matt or texture mode selected | gloss mode selected, still uneven |
Look at the area in low-angle backlight and with a magnifier. A bubble has a clear edge and looks like a cavity. A crater is a spot where the varnish has pulled back in a ring. Trapped dust shows up as a small bump with a particle in the middle.
Diagnosis table
| Symptom | Likely cause | What helps |
|---|---|---|
| Small bump with a particle or fluff | Dust or fibres on the object or from the air | Clean and dust the object, keep the print area clean |
| Small craters, varnish pulls back in a ring | Contamination that disturbs wetting (grease, fingerprints, residues) | Clean with isopropanol, stop handling with bare fingers |
| Bubbles concentrated on wood, MDF or other porous parts | Air or moisture from the pores | Sand, dust, store dry, seal the pores with a base layer |
| Bubbles that increase with more varnish passes | Varnish stays uncured for longer between passes | Fewer layers, keep print time short, keep the room low in dust |
| Uneven, orange-peel-like surface in gloss mode | Varnish cures too early or too strongly, before it levels out | Check gloss mode, try reducing lamp power if the device allows it |
| Fine gaps, lines or dots without varnish | Nozzle dropouts, often caused by air in the varnish line | Nozzle check, cleaning, fill or flush the line |
| Varnish flows poorly, looks thick and uneven | Room or ink too cold | Bring the room to the manufacturer’s value, do not heat the ink directly |
| Surface stays tacky | Varnish not fully cured | Check lamp and lamp glass, maintenance |
Cause 1: Dust, fluff and residues
The most common cause. Every particle on the surface is enclosed by the varnish and becomes visible as a bump, because varnish follows every irregularity. Greasy spots such as fingerprints behave differently: the varnish wets the surface poorly there and pulls back. This is how small craters form. For paint defects such as craters, the Fraunhofer IPA names exactly these two groups: particle inclusions and contamination that disturbs wetting.
What helps:
- Clean the surface with isopropanol and a lint-free cloth before printing, as eufyMake also recommends
- After that, handle it only by the edges
- Peel off protective films only right before printing and remove adhesive residue
- Keep the print area low in dust, and do not do any work producing sanding dust nearby
Mimaki describes a further point for its devices: the longer the clear varnish sits open between application and full curing, the more time dust and air bubbles have to settle, and the less glossy the result is.
Cause 2: Porous materials such as wood and MDF
Wood and MDF have pores that hold air and residual moisture. When liquid varnish lands on them, this air can rise and stay trapped in the varnish as bubbles. Manufacturers of UV-curing resins describe the mechanism like this: air in the pores expands in the heat of curing, and moisture evaporates. Manufacturers of clear UV varnishes also name porous substrates as a typical cause of bubbles and pinholes.
What helps:
- Sand the wood along the grain, finishing with roughly 220 grit, and remove the sanding dust thoroughly
- Store the wood dry and do not print straight from a cold, damp store
- Seal the pores before the varnish. Clear varnish manufacturers generally advise sealing or filling porous substrates. In UV printing, a closed layer of white or a suitable primer does this. Whether that is enough for your wood can only be shown by a test print.
- Coarse-pored woods such as oak are more critical than fine-pored ones. More on this under Printing on wood with UV.
Cause 3: Too many varnish layers or too slow a process
More varnish is not automatically smoother. Every additional layer lengthens the print time and with it the time in which dust can land on fresh varnish. Mimaki gives exactly this as the reason why a second curing pass increases the risk of dust and air bubbles. In addition, every new layer follows the irregularities of the layer beneath.
What helps: For pure gloss and protection, start with few varnish layers and increase only if the gloss is not enough. Many layers are intended for relief and texture, not for a smooth gloss surface. How colour, white and varnish sit on top of each other is explained in Colour, white and varnish.
Cause 4: Varnish cures too early
For varnish to be glossy, it must be able to flow briefly before it hardens. If the lamp cures it immediately and strongly, the droplet structure remains. That is the matt effect, which is a nuisance in gloss mode. Manufacturers of UV curing systems and UV varnishes describe too high a UV output too early as a cause of orange peel, that is, an uneven, slightly wavy surface. These are not true bubbles, but they are often mistaken for them.
What helps: Check that gloss mode is really selected in the software and not matt or texture. On devices that allow you to adjust lamp power or curing delay, try a gentler setting. Always let the varnish cure fully in the process.
Cause 5: Air in the varnish line
In UV printing, air in the ink system does not cause bubbles in the varnish film but nozzle dropouts. In the varnish, this looks like fine lines or dots where varnish is missing. eufyMake names two typical triggers: trapped air after a cartridge change and an ink level below the intake tube. When shaking the white ink, eufyMake also warns not to shake too hard, as air bubbles will form otherwise. Whether and how your varnish should be shaken is given in your device’s manual.
What helps: Print a nozzle check for the varnish channel. Varnish is hard to see on paper, so a test on dark, smooth material or in backlight helps. Then run a cleaning or the fill function for the lines. According to eufyMake, the varnish channel clogs faster than normal colour because varnish is thicker. Care instructions are under Maintaining a UV printer.
Cause 6: Room or ink too cold
UV ink becomes more viscous in the cold. xTool gives a working range of 15 to 35 °C for its UV ink, while eufyMake recommends a room temperature of 18 to 22 °C for its printer and advises against heating the ink directly. Thick varnish levels out worse, and clear varnish manufacturers name excessive viscosity as a cause of an uneven surface. More on ink types and storage under UV ink explained.
Cause 7: Varnish not cured through
If the varnished area stays tacky or can be dented, it has not fully cured. Another layer of varnish on such an area does not make the problem better. One possible cause is weakened curing, for example because dust and ink mist on the lamp glass or reflector reduce the output. Lamp and glass therefore belong to regular maintenance. If the whole layer adheres poorly, the article UV print does not stick helps.
For business use
Bubbles in the varnish are often barely noticeable on photos on wood, but immediately visible on metal, acrylic or dark high-gloss surfaces. For series work, plan a test piece from the same material and the same batch. Cleaning, dusting and a nozzle check before the varnish print take a few minutes and save rejects.
Quick checklist
- Defect or intended matt and texture structure? Check the mode in the software.
- Surface cleaned with isopropanol and not touched afterwards?
- Dust and fluff removed, print area low in dust?
- For wood and MDF: sanded, dusted, dry, pores sealed with white or primer?
- Number of varnish layers reduced to what is necessary?
- Nozzle check printed for the varnish channel, line clear?
- Room temperature within the range the manufacturer gives?
- Varnish fully cured, lamp glass clean?
Continue with: Varnish in UV printing and Printing on wood with UV.
Sources
- Dust and air bubbles with longer curing time of clear varnish: Mimaki Clear Control
- Gloss, matt and embossed printing with clear varnish, number of layers: Mimaki Clear Ink Print Guide (PDF) (no longer online)
- Cleaning, air in lines, shaking, room temperature, varnish channel: eufyMake: UV-Druck Probleme und Lösungen
- Viscosity and temperature range of UV ink: xTool: Ultimate Guide to UV Ink
- Craters, particles and contamination that disturbs wetting: Fraunhofer IPA: Schadensanalyse an Lackoberflächen (PDF)
- Outgassing of porous substrates with UV-curing liquids: Incure: Dealing With Bubbles in Light-Cured Adhesives
- Bubbles, pinholes and orange peel with clear UV varnish: DSN: Common problems with clear UV varnish
- Orange peel from curing too early and too strongly, dirty reflectors: IUV: Surface defects and UV curing control