Pinholes & Outgassing
Why tiny holes, bubbles and craters form in coatings over concrete, zinc primers and other porous surfaces, and how to find, repair and prevent them.
Key takeaways
- Pinholes are tiny holes that extend through one or more coats; outgassing is gas escaping from a porous substrate or primer during cure, which often creates them.
- Concrete, inorganic zinc primers, castings and other porous surfaces are the usual sources of outgassing.
- A pinhole is a direct path to the substrate, so in immersion linings even a few can lead to corrosion or leaks.
- Holiday testing finds pinholes that the eye misses; repair means opening, filling and recoating, then retesting.
Pinholes are minute holes in a coating film, often only fractions of a millimeter across, that may penetrate to the substrate. Outgassing is the release of air, vapor or other gases from the substrate or underlying layers while the coating is still fluid or soft. As the gas escapes it forms bubbles; when they burst after the film has started to gel, they leave craters and pinholes that the coating cannot level out.
Not every pinhole comes from outgassing. Solvent popping, air entrained during mixing or application, moisture reacting with isocyanate-based coatings, and simple low film build over a rough profile can produce similar defects. The cure differs for each, so identifying the source matters.
What pinholes and outgassing look like
- Pinholes: tiny dark points, often visible only under raking light or 10× magnification. On steel in wet service they may show as pinpoint rust spots.
- Bubbles and blisters during cure: intact or burst domes appearing within minutes to hours of application, typical of outgassing from concrete.
- Craters: small bowl-shaped depressions where a bubble burst and partly flowed back.
- Foam or seediness: a fine, uniform bubble texture through the film, often from moisture reacting with polyurethanes or aromatic polyureas, or from air whipped in during mixing.
Craters caused by surface contamination (fisheyes) look similar but usually have a raised rim and a central spot of contamination; they are a wetting problem rather than a gas problem.
Root causes
Outgassing from concrete
Concrete contains air and moisture in its capillary pores. When the slab warms — from sunlight, rising ambient temperature or space heating — that air expands and escapes through the fresh coating. Fast-setting systems such as polyurea are especially prone because they gel before bubbles can escape and level. Open, freshly profiled concrete with exposed pores and bugholes outgasses more than a sealed or primed surface.
Porous primers and substrates
Inorganic zinc silicates and some other zinc-rich primers are inherently porous. A full wet coat applied directly over them displaces the trapped air, which bubbles up through the topcoat. Castings, porous weld metal and some hot-dip galvanized surfaces can outgas under heat, notably during powder-coating cure.
Solvent popping
When a film is applied too thick, or the surface skins quickly because of high temperature, wind or a solvent blend that evaporates too fast, solvent below the skin forms bubbles that burst through and leave pinholes.
Application and mixing
High-speed mixing that draws a vortex entrains air. Rolling with an unsuitable nap or overworking the film can leave bubbles. In airless spray, incorrect pressure, holding the gun too far away (producing a dry, porous film) or air and moisture in supply lines can all cause porosity.
Moisture reaction
Isocyanate-cured coatings react with water to release carbon dioxide. Damp substrates, high humidity or moisture-contaminated components produce bubbles and foam throughout the film.
Low film build over profile
If DFT is too low relative to the surface profile, peaks of the blast pattern can protrude through or be barely covered, producing pinpoint rusting that resembles pinholes.
How to diagnose
- Watch the cure. If possible, observe the first hour after application. Bubbles appearing as the substrate warms indicate outgassing; bubbles present immediately suggest entrained air or solvent.
- Inspect under magnification. Use raking light and a 10× magnifier to classify the defects as pinholes, craters, bubbles or foam, and note their distribution.
- Holiday test. On metal substrates, test following NACE SP0188 or ASTM D5162 — low-voltage wet sponge for films up to 20 mils (500 µm), high-voltage spark testing for thicker films. On concrete, ASTM D4787 covers continuity testing of linings. See holiday testing.
- Section a defect. Cutting through a bubble or examining a cross-section shows whether voids start at the substrate (outgassing), within the film (solvent or air) or throughout (moisture or mixing).
- Check thickness and records. Measure DFT, and review substrate temperature trends, humidity, mixing method and spray settings for the affected area.
Holiday testing voltage must suit the coating type and thickness; excessive voltage can damage a sound film. Use the voltage given in the specification or by the coating manufacturer.
Causes, diagnostic clues and prevention
| Cause | Typical clues | Confirming check | Prevention |
|---|---|---|---|
| Concrete outgassing | Bubbles form as slab warms; worse in sun | Slab temperature trend; voids start at substrate | Prime or seal; coat while slab temperature is falling |
| Porous zinc primer | Bubbles in topcoat over inorganic zinc | Cross-section; primer type | Mist coat/full coat or tie coat |
| Solvent popping | Pinholes in thick or fast-skinning areas | High DFT; hot or windy conditions | Control film build; correct thinner |
| Entrained air | Uniform small bubbles present at once | Mixing and rolling method | Low-speed mixing; correct roller nap |
| Moisture reaction | Foam throughout isocyanate-cured film | Substrate moisture; RH records | Dry substrate; protect components |
| Low DFT over profile | Pinpoint rust at profile peaks | DFT vs. profile measurement | Match DFT to profile; stripe and extra coat |
How to repair pinholes
- Locate and mark. Mark every holiday and visible defect.
- Open and abrade. Sand or grind bubbles open and scuff the surrounding area so the repair coat can bond, particularly if the recoat window has passed.
- Fill. Work a compatible coating or filler into the holes with a brush or squeegee. On concrete, a penetrating primer or a filled scratch coat closes the pores.
- Recoat. Apply a further full coat as specified, timing concrete work for when the slab temperature is stable or falling.
- Retest. Repeat holiday testing after cure to confirm continuity.
Simply spraying another coat over an outgassing surface often reproduces the problem, because the gas source is still there. Seal the pores first.
How to prevent pinholes and outgassing
- Prime porous substrates. Use a penetrating primer or scratch coat on concrete (see concrete surface preparation) and a mist coat or tie coat over inorganic zinc.
- Coat on a falling temperature. Apply to concrete in the late afternoon or evening, or once heating is stable, and shade slabs from direct sun.
- Control film build and evaporation. Stay within the data sheet’s per-coat limits and use the recommended thinner for the conditions.
- Mix and apply correctly. Use low-speed mixing, the correct roller nap and properly set spray equipment with clean, dry air.
- Keep isocyanate systems dry. Check substrate moisture and humidity limits and keep containers sealed.
- Test before service. Holiday test immersion linings and repair every defect.
Frequently asked questions
What is the difference between a pinhole and a holiday?
“Holiday” is the general term for any discontinuity that exposes the substrate, including pinholes, skips, cracks and thin spots. A pinhole is one type of holiday.
Why did my floor coating bubble only on the sunny side?
Sunlight warmed that part of the slab, expanding air in the concrete pores so it escaped through the fresh coating. Applying as the slab cools and priming first usually prevents it.
Are pinholes a problem in atmospheric service?
They are less critical than in immersion, but on steel they still allow corrosion to start and can lead to undercutting over time.
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
