TECHNICAL BULLETIN - 0426TB02 | APRIL 2026

INTRODUCTION

Pin-holing is a common yet aesthetic to critical defect in liquid-applied waterproofing systems. It can compromise membrane integrity, allow moisture ingress, and lead to costly repairs. A clear understanding of its root causes is essential for effective prevention. This bulletin provides a comprehensive guide based on the “Four Core Principles” approach for identifying and addressing the primary causes of pin-holing.

  1. Curing Mechanism of the Membrane
    a. Exothermic Heat Build-up
    b. Solvent or Moisture Release
    c. Reactive Chemistry

  2. Substrate Condition
    a. Concrete Design
    b. High Water-to-Cement Ratio
    c. Poor Compaction
    d. Residual moisture
    e. Concrete Surface Profile (CSP)
    f. Surface Porosity
    g. Contamination

  3. Environmental Conditions
    a. Temperature Swings
    b. Sun Exposure
    c. Barometric Pressure
    d. Dew Point
    e. Air movement

  4. Application Issues
    a. Overmixing
    b. Application Over Wet or Uncured Primers
    c. Thick Coats
    d. Primer not applied in Saturated Surface Dry (SSD) Condition

  5. Guid to Repairing Pinholes in Waterproofing Membranes

  6. Additional Recommendations

  7. Definition of Terms

  8. References

 

1. CURING MECHANISM OF THE MEMBRANE

The way a membrane cures significantly influences the likelihood of pinhole formation.

Exothermic Heat Build-up: Certain waterproofing membranes, especially reactive systems such as epoxies, polyurethanes, and polyureas, undergo an exothermic chemical reaction during curing, generating internal heat as the material polymerises and hardens. This heat increases the temperature of the applied film, accelerating the evaporation of water or solvents. If this occurs before the film has adequately formed, the resulting outgassing can rupture the surface layer, leading to the formation of pinholes and compromising the membrane’s performance.

Solvent or Moisture Release: During the curing or drying process, solvents (in solvent-based systems) or water (in water-based or cementitious systems) evaporate and escape from the applied film. If this release happens too rapidly, due to heat, thick film build, or improper environmental conditions, it can cause outgassing, where vapour pushes upward through the soft, uncured coating. As the coating begins to skin over or cure from the surface down, this trapped vapour has limited escape paths. When it eventually breaks through the surface, it creates tiny craters or pinholes that compromise the film’s uniformity and barrier integrity.

Reactive Chemistry: Isocyanate-rich formulations such as those used in polyurethanes and polyureas can react with ambient or substrate moisture, producing byproducts like carbon dioxide (CO₂) or water vapour during the curing process. In moisture-cured systems, this gas evolution can lead to the formation of internal pressure beneath the film. If the coating begins to skin over before the gases escape, the trapped pressure can rupture the surface, resulting in pinholes or blisters.

2. SUBSTRATE CONDITION

Concrete substrates act like sponges with pores and capillaries that trap air, moisture, and contaminants leading to outgassing when temperatures rise.

Concrete Design: From high water-cement ratios in design mix of concrete and poor compaction of fresh concrete.

High Water-to-Cement Ratio: A higher w/c ratio increases the volume of capillary pores within the concrete matrix. As excess water evaporates during curing, it leaves behind voids, resulting in a more porous surface. This porosity facilitates moisture retention and vapour transmission, which can lead to outgassing during membrane application ultimately causing pin-holing or blistering in the coating.

Poor Compaction: Inadequate vibration or compaction during concrete placement leads to entrapped air and uneven distribution of the concrete mix. This introduces voids, honeycombing, and surface irregularities, which increase the concrete’s permeability and surface roughness. These defects not only compromise membrane adhesion but also create conditions where air and moisture are trapped and released during curing, resulting in pinholes.

Residual Moisture: If present within the substrate at the time of primer or membrane application can result in pin-holing. When moisture is trapped beneath the coating layer, it can vaporise due to ambient heat or exothermic curing reactions. As this moisture tries to escape, it forms small voids or craters commonly referred to as pinholes on the surface of the cured film.

Concrete Surface Profile (CSP): A higher CSP (CSP 4–9), typically produced through aggressive preparation methods such as shot-blasting or scarifying, exposes a greater number of pores, capillaries, and bugholes within the concrete surface. These opened voids can retain air and moisture, which may later migrate upward as vapour during the curing of the applied membrane, resulting in pinholes, blisters, or similar surface defects.

Surface Porosity: Concrete and screed substrates may also contain small to medium-sized voids that, in cross-section, resemble an inverted omega (Ω) shape. These Ω-shaped voids have a narrow surface opening and a wider internal cavity, making them difficult to detect during inspection. When a liquid membrane is applied, it flows into these cavities and displaces the air trapped inside. The displaced air escapes upward through the narrow opening, forming bubbles on the membrane surface, which may burst during curing and leave crater-like defects. These issues arise from the geometry and porosity of the substrate rather than from any deficiency in the membrane itself.

Contamination: Dust, oils, or unremoved laitance can inhibit adhesion and create pathways for trapped gas.

3. ENVIRONMENTAL CONDITIONS

Environmental dynamics significantly affect vapour pressure within the substrate.

Temperature Swings: Rapid or significant temperature fluctuations during or after application can lead to pin-holing in primers or coatings. As temperatures rise, trapped air, moisture, or solvents within the substrate or material can expand and attempt to escape, creating outgassing. If the coating has already started to skin over, these outgassing forms pinholes, small voids or craters on the surface.

Sun Exposure: If a concrete slab is exposed to direct sunlight for an extended period before waterproofing, its surface can heat up significantly. Studies and site measurements have shown that concrete surface temperatures can reach between 50°C and 70°C on hot sunny days, especially in exposed outdoor settings. In contrast, the ambient air temperature may only be 30–35°C. When a membrane is installed over a hot concrete, entrapped air or moisture beneath the coating can expand due to heat. As the membrane begins to cure especially if it’s solvent- or water-based, this expanding vapour tries to escape, forming microbubbles that burst at the surface, leaving pinholes. These pinholes are tiny voids that break the film continuity of the membrane, acting as weak points for future water ingress.

On the other hand, applying waterproofing membrane under direct sunlight can cause the concrete surface to overheat, leading to rapid skinning of the membrane and trapping solvent or moisture beneath. This trapped moisture or solvent expands and forms bubbles, which often burst and create pinholes in the coating. These pinholes compromise the membrane’s integrity and can lead to water ingress, blistering, or delamination over time

Barometric Pressure: Fluctuations in barometric pressure especially falling pressure can contribute to pin-holing during or after the application of primers or coatings. When pressure drops, air or moisture trapped within the substrate or coating material tends to expand and escape in the form of outgassing. If the coating has already begun to form a surface skin, the escaping moisture can create pinholes or small craters in the film.

Dew Point: If a liquid membrane is applied when the concrete surface is at or near the dew point, invisible moisture may be present on the substrate. This condensation layer interferes with adhesion and becomes trapped beneath the membrane. As the membrane cures and heats, the trapped water turns to vapour, forming pinholes or blisters as it escapes.

Air Movement: When air moves quickly across a concrete surface, it lowers the surface vapour pressure, encouraging moisture and air from within the substrate to rise. This accelerated movement of air and vapour from the substrate increases the risk of outgassing during or after membrane application. As vapour escapes through the curing membrane, it can rupture the film, forming pinholes or blisters.

4. APPLICATION ISSUES

Improper application is a leading cause of surface defects like pinholes.

Overmixing: Excessive mixing or high-speed agitation during primer or coating preparation can introduce excessive air into the material, resulting in entrapped air bubbles. When the product is applied, these bubbles may rise to the surface and burst during curing, leaving behind small cavities or pinholes.

Application Over Wet or Uncured Primers: Applying subsequent layers over primers that are still wet or not fully cured can result in pin-holing. When a membrane, coating, or second primer layer is applied prematurely, trapped moisture or solvents from the underlying uncured primer can vaporise and form small air pockets as they try to escape. This outgassing process causes pinholes, tiny voids or craters in the film of the newly applied layer.

Thick Coats: If the primer is applied in an excessively thick coat, which can trap air or solvents during curing and can result in pin-holing. When the material is applied beyond the recommended film thickness, it can skin over on the surface while the underlying layer remains wet or volatile. This condition can cause solvent entrapment, outgassing, or blister formation, which appear as pinholes once the primer dries.

Primer not applied in Saturated Surface Dry (SSD) Condition: When a surface is too dry or too porous, it can draw the moisture or solvent out of the primer too quickly, resulting in incomplete film formation. This condition often leads to the development of pinholes, small, crater-like voids in the primer layer that compromise the continuity and integrity of the subsequent coating or membrane.

5. Guide to Repairing Pinholes in Waterproofing Membranes

Applicable to Water-Based Acrylics or Polyurethanes, SBR, Solvent-Based, and Cement-Based Systems

A. Surface Preparation (For All Membrane Types)

Proper surface preparation is critical to ensure effective and durable pinhole repairs.

Assess and Identify

  1. Determine the underlying cause of pinholes (e.g. trapped air, residual moisture, substrate outgassing, or improper application).
  2. Visually mark all affected areas for treatment.

Clean the Surface

  1. Use a clean brush, cloth, or compressed air to remove dust, dirt, and loose debris. In the presence of inverted omega holes, pinholing can be prevented by grinding the concrete surface to open the holes before priming and coating, or using a low-viscosity or deep-penetrating primer to bridge or partially close these cavities, or applying the membrane in the late afternoon when lower temperatures reduce the air volume inside the concrete and help minimise pinholing even if some omega holes remain.
  2. Where necessary, clean the area using a damp cloth or Pasco Aquawipes.
  3. Allow the substrate to dry completely prior to applying any repair compound.
  4. For greasy or contaminated surfaces, particularly in solvent-based or bitumen-based systems, clean with a compatible solvent such as Pasco Xylene.

B. Repair Based on Membrane Type

1. Water-Based Acrylic, SBR & Polyurethane Membranes

Repair Procedure:

a. Surface Preparation and Filling

  • Lightly abrade the surface to expose the pinholes.
  • Fill the defects using a compatible hybrid sealant, such as Pascoflex FS, ensuring complete filling and good adhesion.

b. Reapplication of Membrane

  • Apply the original water-based PU membrane over the repaired area using a brush or roller.
  • Apply in thin, even coats to prevent entrapping air or moisture.
  • Feather the edges and ensure a uniform, continuous film.
  • Confirm that the total dry film thickness (DFT), including the repaired area, meets the manufacturer’s specification.

c. Curing

  • Allow the membrane to cure approximately 3 days (72 hours)*, or as specified in the product’s technical datasheet.

    *At 23°C and 50-60% RH, as site conditions may vary and can affect the curing time of the membrane.

2. Solvent-Based Waterproofing Membranes
(e.g. Solvent-based PU, bitumen-based systems)

Repair Procedure:

a. Safety and Ventilation

  • Ensure the work area is well-ventilated.
  • Always wear appropriate PPE when handling solvent-based materials.

b. Surface Preparation and Filling

  • Clean and lightly abrade the pinhole area.
  • Use a compatible polyurethane sealant such as Pascoflex PU25 to fill pinholes. Allow full cure before recoating.

c. Surface Reactivation

  • Some solvent-based membranes require surface activation before recoating.
  • Use Pasco Xylene for reactivation, or for best performance, apply Aquaprime PVC Primer as per manufacturer guidelines.

d. Reapplication of Membrane

  • Apply the same solvent-based membrane over the cured sealant using a brush or trowel.
  • For deeper pinholes or minor voids, use a filler or paste, such as additional Pascoflex PU25, prior to recoating.
  • Ensure the repaired area achieves the required DFT in accordance with the product specification.

e. Curing

  • Allow the membrane to cure approximately 3 days (72 hours) *, or as specified in the product’s technical datasheet.

    *At 23°C and 50-60% RH, as site conditions may vary and can affect the curing time of the membrane.

3. Cement-Based Waterproofing Membranes

Repair Procedure:

a. Pre-wetting the Surface

  • Lightly dampen the repair area prior to application.
  • Avoid surface pooling or standing water.

b. Filling and Patching

  • Use a repair mortar such as TecPatch FC or a suitable cementitious waterproofing compound like Aquaproof 121.
  • For improved performance in critical areas, consider epoxy-based products like Megapoxy PM.
  • Apply using a putty knife or trowel, pressing the material firmly into pinholes.

c. Reinforcement (if applicable)

  • For large or clustered pinholes, embed Pasco Type S Fabric into the first wet coat of membrane to reinforce the repair zone.
  • Fully saturate the mesh to ensure strong adhesion and integration.

d. Recoating

  • After the patch has achieved its initial set, apply one to two coats of the cementitious membrane over the repaired section.

e. Curing

  • Maintain a moist curing environment if required, and protect the area from rain, direct sunlight, and premature drying during the curing process. Allow the membrane to cure approximately 3 days (72 hours) *, or as specified in the product’s technical datasheet.

    *At 23°C and 50-60% RH, as site conditions may vary and can affect the curing time of the membrane.

6. Additional Recommendations

  • Adhesion Testing: Conduct pull-off adhesion test if performance assurance is required post-repair.
  • Post-Repair Inspection: Reinspect the repaired area approximately 3 days (72 hours)* after application to confirm effectiveness.
    * At 23°C and 50-60% RH, as site conditions may vary and can affect the curing time of the membrane.
  • System Compatibility: Always use repair products that are compatible with the original membrane system.
  • Preventive Measures: If pinholes are widespread, consider applying a primer coat or recoating the entire area, as this may indicate substrate porosity or inconsistent application technique.

7. DEFINITION OF TERMS

Barometric Pressure

Barometric pressure, also known as atmospheric pressure, is the weight of the air exerted on a surface by the Earth’s atmosphere. In waterproofing membranes, barometric pressure can influence the curing process and outgassing behaviour, particularly in liquid-applied membranes.

CSP

Concrete Surface Profile (CSP) refers to the degree of roughness or texture of a concrete surface, which directly affects the mechanical bond between the substrate and the waterproofing membrane.

Dew Point

The dew point is the temperature at which air becomes saturated with moisture and condensation forms. If the substrate temperature falls to or below the dew point, water vapour condenses on the surface—even if the air feels dry.

Inverted Omega Holes

Inverted omega holes are tiny, often hidden surface cavities in concrete or screed that have a cross-section resembling an upside-down Greek letter omega (Ω). Instead of being simple open pores, these cavities have a narrow opening at the surface and a wider void underneath—like a small pocket or chamber beneath a thin surface layer.

Outgassing

Outgassing in waterproofing refers to the release of trapped air or moisture vapour from a concrete or substrate surface after a waterproofing membrane has been applied. This phenomenon can create pinholes, blisters, or bubbles in the membrane, compromising its integrity.

Relative Humidity

Relative Humidity (RH) in the context of waterproofing membranes refers to the amount of water vapour present in the air (or within a substrate) compared to the maximum amount the air can hold at a given temperature, expressed as a percentage (%).

SSD Condition

Saturated Surface Dry (SSD) condition refers to a state of a concrete or cementitious substrate where the surface is moist but free of standing water. The pores within the substrate are filled with water, but no water film is visible on the surface.

Vapour Pressure

Vapour pressure is the pressure exerted by water vapour molecules present in the air or within a substrate (such as concrete). It represents the tendency of moisture to evaporate or move as a gas from a liquid or solid phase into the atmosphere.

8. REFERENCES:

Australian Institute of Waterproofing. Outgassing of Concrete Substrates. Technical Bulletin, 1 June 2025.

Neogard, a part of Hempel. Troubleshooting Guide: Causes, Repair & Prevention. March 2020.

Polycoat Products. Technical Bulletin #1: Pinholes & Bubbles Outgassing. September 2016.

Tecnopol. Pin-holes: What They Are and How to Prevent Them. n.d., https://www.tecnopolgroup.com/news-and-updates/pin-holes-what-they-are-and-how-to-prevent-them.

The Euclid Chemical Company. Coating Defects Caused by Concrete Outgassing. Technical Bulletin CP 03.

Tremco Construction Products
Group. Technical Service Bulletin No. P 1 19: Out gassing of Concrete Substrates. n.d.

W.R. Grace & Co. Out-gassing in Substrates Causing Blistering & Pin-holing of Fluid Applied Waterproofing Membranes. Technical Bulletin.

 

DISCLAIMER:The information contained in this Technical Bulletin is provided in good faith and is based on the Pasco Construction Solutions Pty Ltd.’s current knowledge, experience, and testing. It is intended as a general guide to assist in the proper selection, installation, and application of waterproofing systems under typical site conditions in Australia.While every effort has been made to ensure the accuracy of the information, the manufacturer makes no warranties or representations, express or implied, regarding the completeness, accuracy, or fitness of the material for any particular purpose. Product performance can be influenced by a wide range of factors including, but not limited to, substrate condition, application method, site climate, and workmanship.It is the responsibility of the user, specifier, or installer to verify the suitability of the product for the specific project and to ensure compliance with all relevant Australian Standards (such as AS 3740, AS4858, AS 4654.1, and AS 4654.2), building codes, and regulations.This document is subject to revision without notice. For the latest version or further technical advice, please contact the Pasco’s technical support team.