November 22, 2021

Common Causes and “Cures” for Blisters in Polyurethane Coating Systems

Pecora is a major supplier of Deck Coating Systems for vehicular and pedestrian applications. Millions of square feet have been waterproofed worldwide using these systems since 1990. Our customers have enjoyed expert technical service, architectural and engineering consultation, and a single source for their waterproofing needs. Deck Coatings consist of multiple layers of fluid applied compounds that cure to a tough elastomeric, resilient, waterproofing coating. Pecora Deck Coatings Systems are systems of polyurethane coatings; each designed to provide specific properties that enhance the quality of the total system. When properly designed and formulated, these coatings are engineered systems that prevents water from entering the surface to which they are applied while providing a safe (nonslip) surface with pleasing aesthetics. Because the Pecora Polyurethane Coating Systems are positive seal or vapor barrier systems, blistering may occur during application if certain conditions are present. In this blog post, we will discuss the three types of blisters and how to handle these issues.


  1. Moisture Blister

Normally, blisters caused by moisture are fairly large (quarter to silver dollar in size). They have a trace of moisture under them when cut open. These types of blisters can be caused by coating a damp or wet deck. The most important aspect to all systems is the bond of the coating to the substrate. All precautions should be taken to assure that the deck to receive the Pecora Polyurethane Deck Coating System is thoroughly dry. Moisture blisters will also occur between layers of coating if the deck is damp between coats (due to rain, dew, fog, etc.) There may or may not be a trace of moisture under these blisters. The moisture between coats leaves a definite watermark on the back of the blister itself.

To minimize the moisture blister problem after it occurs, you must cut out these blisters and leave the void open in order to allow the moisture to escape. After the moisture has escaped and the surface has dried, make necessary repairs.


  1. Pinhole Blister

Normally, pinholing occurs when a concrete deck has been sandblasted, shot-blasted, ground, or if the new concrete has not been properly finished with a steel trowel. The surface of the concrete has been broken, opening up air pockets in the concrete which are actually tiny holes in the concrete about 1/8″ to 1/4″ deep. As you coat the deck, the coating will bridge these holes, and as the film dries, small blisters will form as the air inside the pinhole expands. If you cut these blisters open, there will be a tiny hole in the deck. While the coating is still in the liquid stage, these blisters must be broken with a squeegee or stiff broom before another coat is applied.

Several measures have proven effective when handling these small pinhole blisters:

  1. Slightly increase the amount of primer being used or apply two light applications of primer.
  2. Apply the specified system in thinner coats. Obviously, this procedure means more coats are involved in applying the full system.
  3. Begin application of first base coat late in the day, after the heat of the day has passed and after the deck has begun to cool. This will allow the base coat flow into the pinholes and plug the pinholes upon curing.


  1. Application Blisters
    1. Gassing can occur when the coating system is applied at an application rate or thickness which is greater than that in the recommended application instructions. It may occur anywhere in the coating film. When it occurs at the top surface of the cured film, gassing resembles a small pinhole. To reduce the occurrence of this problem, apply the material in thinner coats.


***Pecora-Deck HB1000 Industrial Coating is a 100% solids, chemically curing, high build coating system which will eliminate blisters as a result of over application***


    1. Solvent blisters occur when a coat of material is applied before the preceding coat has cured. Solvents in the uncured coat will not have had sufficient time to leave the film and are trapped by the next coat. Solvent blisters are most common during marginal weather conditions such as low temperatures, low humidity, or both. These blisters are usually fairly large (quarter-sized up to several inches) and will be tacky on the back side of the film. To repair, cut out the blisters and allow the entire film to dry before touching up and recoating.


For more information or deck coating application recommendations, we urge you to contact our Pecora Corporation Technical Services Group. Our expert team is highly qualified to answer any question, recommend the proper solution, or provide testing training. Our Technical Services staff offers training in product technology and use and provides the technical assistance you require in the planning and implementation of your project. They can be reached by email or by phone (800)-523-6688.


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