blistered coating

Problem Solved: Taking Control of Preexisting Conditions

Specifications for maintenance painting often state that the commencement of cleaning and painting is an acknowledgment that the contractor accepts the existing building conditions as being suitable for painting and, implicitly, suitable for the performance of the paint system.

As seen in Durability and Design Summer 2017 Issue

This article addresses a requirement in specifications that contractors might easily overlook. Here’s an example from a commercial painting specification: “The Contractor shall examine the substrates and areas to be painted, and identify conditions requiring repair prior to the start of cleaning and painting.” The specification continues, “The start of cleaning and painting shall be construed as acceptance of the surfaces and conditions by the Contractor.”

So, to reduce the possibility that this stipulation could come back to haunt them, what types of things should contractors examine before beginning their work and thereby “accepting” the conditions?
In maintenance painting, there are several existing conditions that should be examined and discussed with the owner/architect so that:

  • Repairs can be made by others;

  • The scope of work is changed so the contractor can address them; or

  • The items are excluded from the contractor’s responsibility.

The existing conditions and how they are addressed will depend on the contractor’s scope of work, the contractor’s ability to expand the scope through a change order, and the requirements of the specification, but a few items to consider before starting the work are provided in the following sections.

To put the items in context, the work that is the subject of these examples involves cleaning and painting the exterior of a previously painted building. The predominant construction materials are split-faced block and scored block. The existing coating exhibits scattered blistering and peeling. The specification requires the contractor to pressure wash the surface to remove loose paint, spot prime exposed block and apply two full coats of waterborne acrylic to the entire surface.

ADHESION OF THE EXISTING COATING

Since the job involves maintenance repainting, the existing coating serves as the foundation for the newly applied material. If the existing coating is not sound, the performance of the new coating will be jeopardized. Even if the newly applied coating adheres well, and future peeling only occurs within the existing coating, the contractor is at risk of doing repairs. The owner will simply argue that the cleaning performed by the contractor was inadequate to locate and remove the preexisting poorly adherent coating.

Adhesion tests provide valuable information regarding the suitability of the existing coating to be overcoated. Coatings eventually become too old, too heavy or too weak to withstand the rigors of surface preparation, or to support the weight and curing stresses of additional coats.

The application of more coats to a weak foundation leads to peeling, so the integrity of the existing system and its capacity to support additional material should be determined before the work begins.

x cut coating

Fig. 1: Probing of the “X” cut through the coating reveals poor adhesion between the coating layers. If overcoated, there is a reasonably high risk that peeling will occur within the existing system.

blister coating preexisting conditions

Fig. 2: Even without measuring the thickness of the coating in this blistered area, it appears to be heavy, and should trigger a thickness evaluation.

If the adhesion appears to be suspect, it should be reported to the owner/architect. Testing might reveal that the contractor will need to remove more coating than was anticipated when the bid was prepared, or that none of the existing coating is durable enough to be salvaged

blistering wrinkling coatings moisture

Fig. 3: Blistering and wrinkling of newly applied coating at mortar joints due to moisture.

These issues should be identified in advance so the appropriate action can be negotiated before the work begins. ASTM D6677, Standard Method for Evaluating Adhesion by Knife, describes a simple test that involves the use of a knife to cut an “X” into the coating and probing the intersection of the scribes to determine its adhesive strength (Figure 1).

COATING THICKNESS — REDUCTION IN COATING PERMEANCE

Other problems can occur when the underlying layers of paint are heavy. As new coats are applied, the potential to entrap moisture vapor within the wall increases in certain climates due to the reduced permeance of the system. In our example building, the potential for this to occur is even greater because the specification requires the application of two full coats over the old paint across the entire building.

The threshold of concern for coating thickness depends on the coating type, age, permeance and geographical location. The location and use of the building will dictate whether the air/vapor movement through the walls is from the inside out or the outside in, and therefore the corresponding tolerance for reduced permeance.

If many coats are present and the film seems to be heavy (Figure 2), it should be pointed out so the architect or coating specialist can determine whether recoating is a viable maintenance strategy or whether the coatings should be completely removed and replaced. Even if the scope of work remains the same, the owner/architect has at least been made aware that the thickness of the system could lead to the entrapment of moisture vapor within the wall assembly.

DAMP WALLS

Visual assessments for evidence of moisture (such as efflorescence, lime run or moisture-filled blisters) and moisture tests should be conducted to determine if there is a preexisting moisture problem in the building walls. Sources of moisture should be identified and corrected before the painting work begins. A damp wall can affect the performance of the coating by causing wrinkling and blistering (Figure 3). It also can impact the schedule if extra time is required to allow the wall to dry, which in some cases may not occur naturally (for example, if the masonry-fill insulation is wet). For many reasons, ranging from productivity to quality, preexisting dampness in walls should be identified and reported.

CONTINUITY OF THE EXISTING COATING

Another common requirement in specifications involves the continuity and appearance of the film after application. For example: “All coatings shall be free from misses, skips, shadow-through, runs, sags, lap marks, roller nap, brush strokes, and similar irregularities. Coating applied to CMU and concrete surfaces shall be virtually pinhole-free which is defined as no more than 20 pinholes per block, or block size equivalent (144 in.2).”

pinholing coating pinhole

Fig. 4: Heavy pinholing of the existing coating can create challenges in meeting the pinhole requirements for overcoating.

Complying with these requirements for maintenance painting can be difficult since the continuity and quality of the existing coating can directly affect the continuity and quality of the new coating. In the case of split-faced block, it is not uncommon to find a lot of pinholes in the existing coating, which can be challenging to correct (Figure 4). Without an additional primer application and/or a great deal of back rolling and working the coating into the surface, the existing pinholes can telegraph through the new coating, failing the pinhole criteria.

If the scores of scored block are covered with runs, drips, bridged coating and sags (Figure 5), it will not be possible to thoroughly apply the new coating to completely cover all surfaces of the scores. In addition to poor aesthetics, these deficiencies serve to trap moisture in the scores, providing a direct path for rainwater to saturate the block through the uncoated faces, which can lead to blistering and peeling.

Past application deficiencies like this should be pointed out since the only way to correct them is to grind out the scores to remove the existing coating and completely apply a new coating system.

paint removal

Fig. 5: Existing paint covering the scores of scored block should be completely removed to allow the new coating to protect all surfaces of the scores, as well as for aesthetics.

Correcting poor workmanship in the existing coating can lead to additional work on the part of the contractor to provide a system that complies with the specification.

OTHER ITEMS

Other items that may be relevant, depending on the scope of work, include concerns such as: the condition of the sealants, cracked or damaged block, mortar joints requiring tuckpointing, defective flashing, cracked or deteriorated EIFS, metal loss or perforation due to corrosion, and presence of obstacles that obstruct access to the work, among others.

SUMMARY

A few basic examinations of the existing coating and the building conditions help to point out issues that can directly affect the nature and amount of work to be performed, as well as the potential impact on coating performance. Moisture content and coating thickness, adhesion, and continuity have been discussed as they serve as the foundation for the contractor’s work, and their suitability is critical to achieving long-term coating performance.

The identification and reporting of potential problem areas before beginning the cleaning and painting work also help to mitigate repercussions from specifications that state, “The start of cleaning and painting shall be construed as acceptance of the surfaces and conditions by the Contractor.”

ABOUT THE AUTHOR

ken trimber kta

Kenneth A. Trimber, president of KTA-Tator Inc. has more than 40 years of experience in the industrial painting field. A NACE-certified Coating Inspector and SSPC Protective Coatings Specialist, he holds a bachelor’s degree from Indiana University of Pennsylvania. A past president of SSPC, Trimber is chairman of the association’s Commercial Coatings Committee, Surface Preparation Committee, and Containment Task Group, as well as a member of the Standards Review Committee. He is a past chairman of ASTM D1 on Paints and Related Coatings, Materials, and Applications and author of The Industrial Lead Paint Removal Handbook.

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