漆Urushi Archive

Reported practice · r/Urushi

Diagnosing wrinkling and cratering in glass-urushi application

A practitioner applying glass urushi directly to untreated glass reports heavy wrinkling and crater-like spots that are worse than with other lacquer types, and asks how to cure the material properly and where to find reliable instructions.

This page is written from one discussion thread. It records what practitioners reported and what they linked it to — not documented workshop practice. Every claim below links to the comment it rests on, so you can read it in context.

Safety

Raw urushi (ki-urushi) is a potent skin sensitiser; contact can trigger a delayed urushiol-type dermatitis that appears 24–72 hours after exposure. Wear nitrile or latex gloves, avoid skin contact with the wet lacquer, and wash any splashed area with copious water and soap immediately.

What the thread establishes

The answer.

The thread converges on two distinct failure modes rather than one. Wrinkling is attributed to applying too thick a layer combined with too-high ambient humidity during the initial set; craters are attributed to surface contamination (a dust particle or a residue on the brush). The practical fix is to let the freshly applied coat set in a low-humidity environment (roughly 50–60 % relative humidity) for a day or two before transferring the piece to a full furo, and to keep the layer thin. For long-term adhesion on bare glass, a cure of three to five weeks is recommended even after the surface appears dry. Glass urushi should not be diluted because it already contains additives whose balance is disrupted by added solvent. Wiping the glass with alcohol before application removes oil contamination that can contribute to cratering.

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What goes wrongWrinkling appears when the coat is too thick and the surrounding humidity is too high during the initial set; craters (small pitted spots) appear when a contaminant—dust, brush residue, or oil—sits on the surface as the lacquer begins to skin over. The two defects look different and have different causes, but a practitioner may lump them together as 'bubbles and spots.'

What recovers itPreventive rather than corrective: apply a thinner coat, ensure the glass is alcohol-wiped and dust-free, and allow the piece to rest in a low-humidity, partially covered environment for one to two days before moving it into the full furo. If craters have already formed, the affected area would need to be sanded back and re-coated, though the thread does not detail that repair.

02

Apply a thin coat of undiluted glass urushi

Use the lacquer at the viscosity it ships in; do not add solvent, because the product already contains additives and dilution can produce unpredictable results. Keep the layer thin to avoid brush-stroke visibility and to reduce the risk of wrinkling.

the comment this came from ↗
03

Initial set in low humidity

Cover the piece loosely (for example with an inverted cup or bowl that does not seal completely) so dust is excluded but air can circulate. Allow the coat to set for roughly one to two days at ambient humidity in the 50–60 % range before transferring to the furo.

the comment this came from ↗the comment this came from ↗
04

Full cure for adhesion

Once the initial set is complete, move the piece into the furo for the full cure. On untreated glass, allow three to five weeks for adequate adhesion even after the surface looks dry.

the comment this came from ↗
··Reported

What was reported

  • Glass urushi applied directly to untreated glass requires a cure of three to five weeks for reliable adhesion, even when the surface appears dry. ↗
  • Glass urushi should not be diluted; it already contains additives, and adding solvent can produce unpredictable results. ↗
  • Wrinkling in a fresh coat is caused by a combination of excessive layer thickness and too-high ambient humidity during the initial set. ↗
  • Craters (small pitted spots) are caused by surface contamination such as a dust particle or brush residue, not by humidity or thickness. ↗
  • A low-humidity initial-set environment of roughly 50–60 % relative humidity, with partial cover to exclude dust while allowing airflow, is the appropriate first stage before full furo curing. ↗↗
  • Wiping the glass with alcohol before application removes oil contamination that can contribute to surface defects. ↗
  • The practitioner's instinct to use a thicker coat to hide brush strokes is counterproductive and increases the risk of wrinkling. ↗↗

Limits of this source

What the thread does not answer.

  • What is the optimal humidity range inside the furo for glass urushi once the initial low-humidity set is complete? The thread references an external article but does not state a specific target.
  • How should a practitioner manage the tension between applying a thin coat (to avoid wrinkling) and achieving a uniform surface free of visible brush strokes?
  • Are there differences in behaviour between glass urushi from different processors or brands that affect cure time or defect susceptibility?
Read the documented methods →