Source 1: outgassing from concrete
Concrete is full of air-filled pores. When the slab warms, that air expands and escapes upward through the wet coating, leaving pinholes or craters as the film sets. It is worst on sunny decks, in the morning and on porous or freshly ground surfaces.
- Apply when slab temperature is stable or falling (late afternoon or evening), not rising.
- Use a penetrating primer or scratch coat to seal pores first.
- Back-roll or squeegee the primer to work it into the surface.
- For spray polyurea, a fast-set primer or a thin “mist coat” before full build can help.
Source 2: CO₂ from moisture
Isocyanates react with water to form an amine and carbon dioxide. In polyurethanes and, to a lesser extent, polyaspartics, moisture from a damp slab, humid air or wet pigments and solvents can create bubbles and foaming.
- Test concrete moisture (ASTM F2170 or F1869) and use moisture-tolerant primers where needed.
- Keep the surface above the dew point and drums sealed.
- Formulators add moisture scavengers such as molecular sieve or PTSI.
Source 3: solvent popping and entrained air
In solvent-borne coatings, a surface that skins over too fast traps solvent, which later bursts through as pops or pinholes. Heavy film builds, hot surfaces and fast solvents make it worse. Air whipped in by aggressive mixing or rolling can also leave bubbles.
- Respect maximum film thickness per coat and recoat times.
- Use the specified, slower thinner in hot conditions.
- Mix at low speed and let air release before application.
| Symptom | Most likely cause |
|---|---|
| Pinholes over concrete that appear as the sun comes out | Outgassing |
| Foamy, spongy bubbles in a urethane over damp concrete | Moisture → CO₂ |
| Small craters in thick solvent-borne coats | Solvent popping |
| Bubbles visible right after rolling | Entrained air |