overcoating failure

Overcoating: Maintenance or Mayhem?

Whether a bridge, tank, pipeline, vessel, manufacturing facility, architectural structure or amusement park ride, at some point there will be a need for maintenance painting. The “paint guy” will share responsibility with those involved in general maintenance, funding, prioritizing, public relations and safety.Those familiar with maintenance painting may recall the four maintenance painting strategies:

Do nothing: The structure, including the coating system, is in good or very good condition. Maintenance painting is simply not necessary or needed. A second “do nothing” option involves the “extreme condition,” when the coating and corrosion breakdown of the structure or item is so poor that it will be demolished, retired or decommissioned. Spot repair: Cleaning and coating only a few areas or sections of a structure will enhance corrosion protection, is relatively inexpensive and the aesthetics are not a major consideration. Localized areas are cleaned and spot rust and degraded or poorly adhered coatings are removed. One or more protective coating layers are applied to the area.

Spot repair and overcoat: Coating deterioration and breakdown in one or more of the existing coats is providing some protection from corrosion, but erosion, weathering or other environmental conditions have compromised the barrier protection and/or detracted from the desired aesthetic condition. Overcoating may be applied to the entire surface or be limited to specific areas such as those visible to the public.

Remove and replace: This maintenance option is employed when existing coating systems need to be entirely removed and replaced due to extensive breakdown and corrosion of the structure or item. Depending on the service environment and other considerations, this approach may apply to all or only sections (zones) of the surface area.

This column focuses on overcoating, which nearly always includes some degree of spot repair and touch-up application. Whether to enhance corrosion protection, restore and improve aesthetics or both, pre-planning and providing effective, straight-forward specifications are critical to success. Considerations appropriate for overcoating, or any other scope of maintenance painting for that matter, are briefly described simply to provide some context for determining if overcoating is a reasonable maintenance option. Recommendations for project specifications follow these considerations.

Pre-Planning: Maintenance painting is most frequently triggered by some visual condition, be it corrosion and rusting, loss of gloss or color and at times for regulatory purposes (safety color codes). This may result in execution of a maintenance plan or program that already exists (for example, in petrochemical facilities), or alert the owner that maintenance painting will be needed, and planning and preparing work orders or contracts should be initiated. The impact on operations, safety and access are evaluated to determine the possible implications that may be imposed by maintenance painting activities, including shutdown times, lane restrictions on bridges and contractor access.

Fig. 1: Coating compatibility is a bigger problem for overcoating than many believe. All photos courtesy of the author.

Coating Condition Assessment: A coating condition assessment may be limited to a visual assessment or involve a closer inspection of the coating system. The physical and chemical characteristics of the existing coating system, including adhesion, coating layers and thickness, degree of rusting or corrosion, topcoat resin type and the presence of heavy metals (e.g., lead or chromium) may all be measured or evaluated when extensive assessments are conducted. Refer to to SSPC “Technology Update No. 3, Overcoating” (SSPC-TU 3) for discussion on the use and possible interpretation of such collected data. When overcoating appears to be a viable option for maintenance painting, the use of test patches to evaluate surface preparation and candidate coatings should be considered early in the project planning phase.

Project Planning: The scope of the maintenance painting to be performed may be established based on historical experience, the coating condition assessment (general or detailed), available funding or test patch and other testing results. However, once the scope has been established, the details of the project and its impact on operations, the public, budget, duration, season, timing and details for execution can be used to establish the details for contracts, work orders, special provisions and specifications.

Fig. 2: Edge coating should include a stripe coat, even for spot repairs.

Any additional aspects of the overall scope of the project should be detailed. This may include repair or replacement of steel elements or components, identification where specific procedures may be required (coating tie-ins), remediation of soluble salts, specific cleaning agents or processes to neutralize or sequester surface contaminants, and limitations on the surface preparation and/or application procedures permitted.

When abrasives are used, restrictions, such as the use of silica sand, should also be considered. Safety, health and environmental issues are also part of the project planning. Lead paint is lead paint, whether it is on a process vessel, bridge or water tank. Compliance with all regulatory requirements must be included in planning, as well.

Seemingly mundane details can escape consideration if not included in the detailed plan, such as who will supply power, compressed air, water, scaffolding and controls access (contractor or owner).

Contract Documents and Work Orders

Fig. 3: Overcoating does not need to be a “one-and-done” strategy.

Preparation of the contract or work order documents should include general and specific conditions, responsibilities of the contractor, owner and other parties that may be part of the agreement. Insurance, bonding, definitions, schedule and time allotment are also included. The key component of interest here are the technical specifications for overcoating that are a part of the contract requirements. If you intend on using an existing maintenance painting specification, make sure that it is up-to-date and not simply “copy-and-pasted” pieces of specifications or based strictly on coating manufacturers’ recommendations. Because the specifications are contractual, a critical review of the draft specification should always be performed by someone with the experience and credentials to do so. This can be the internal “paint guy (or gal),” a trusted vendor or a consultant.

Technical Specifications for Overcoating

Ultimately, in the eyes of a contractor, the responsibility for the specification will rest with the owner, and rightly so, as the specification is part of the owner’s contract documents. When you are tasked with preparing a specification, whether you work for or are retained by the owner, where do you think the content responsibility will lie in the eyes of the owner?

Correct! The engineer, consultant or coating manufacturer that prepared them. The quality of the contract and specifications can contribute to a relatively smoothly running project or can lead to Mayhem (you know, that guy from the insurance commercials).

Specification for Overcoating Structural Steel

General: The general section of the specification should include the scope of work to be performed; definitions for words and terms used in the specification; a list of published reference standards, test methods, procedures and regulations and other pertinent informational documents relevant to the work. Safety requirements that the contractor is responsible for, as well as the qualifications of the contractor and personnel may be included here unless addressed elsewhere in the contract. Project documentation and submittal requirements should also be included here.

Fig. 4: Spot repairs on the underside of this bridge seemed sufficient, but later proved to be unsatisfactory.

For the purpose of this article, the scope will involve a small truss bridge where rust is to be removed, tightly adhered coating is allowed to remain and the surfaces are to undergo spot repairs and overcoating.

Materials: The materials section allows the owner or specifier to establish the quality, type and source of materials used for the project. Among the items that may be included in this section are abrasives (required, permitted or excluded); cleaning agents (soaps, alkaline, degreasing); water (potable, non-potable); additives (chloride remediation, corrosion inhibitors); equipment (type, size) and, of course, coatings. Proprietary or similar products that are to be used should also be listed. If substitutions or alternates will be considered, state what documentation or reasoning must be submitted with the request for substitution. The general rule used when selecting an overcoat product is that the same resin can be applied over the existing resin. Use of test patches can confirm compatibility. Tie coats of penetrating epoxy sealers or a “universal” primer may be needed and also evaluated by test patch.

Execution: The execution section provides the detail regarding pre-cleaning requirements (pressure washing); surface contaminants (diesel fumes); surface imperfections (weld spatter, sharp corners, steel defects); items to be protected, and control and containment of the work area. Certain procedures or processes can be excluded (including prohibiting abrasive blast-cleaning) but otherwise leave the means and methods to the contractor. Use performance-based language for surface preparation, coating application, quality control and repair procedures. Note that it is a responsibility to state what is to be done, but ensure there is a performance and/or acceptance criteria and state what methods or standards will be used to measure and test.

Pre-clean all surfaces to be prepared and painted with low-pressure water at no less than 4,000 psi to remove surface contaminants. Prior to pressure washing, remove bulk deposits of dirt, soil, leaves and bird droppings manually. Place all collected debris in containers for disposal. Supplement cleaning with brushes, brooms, rags and wiping to produce no less than a chalk rating of 8 on remaining coating when measured in accordance with ASTM D4214, “Standard Test Methods for Evaluating the Degree of Chalking of Exterior Paint Films.”

Surface Preparation Spot Repairs

Clean all areas of visible rust and corrosion in accordance with SSPC-SP 15, “Commercial Grade Power Tool Cleaning.” Remove pack rust and crevice corrosion using impact tools to the satisfaction of the project engineer. Prepare areas of blistered, cracked and delaminated coating in accordance with SSPC-SP 3, “Power Tool Cleaning.” Surface temperature should be a minimum of 5 F above the dew point before performing surface preparation. Spot-abrasive blast-cleaning to SSPC-SP 6/NACE No. 3, “Commercial Blast Cleaning” would only be justified if there were numerous spots or locations where the size of spots were quite large or abundant. In such an instance, full removal and replacement may be a more desirable maintenance option.

Intact, adherent coating surrounding rusted and/or degraded areas must be feathered back such that the repair coating will extend at least 2.5 to 5 centimeters (1 to 2 inches) onto the intact coating.

Soluble Salt Mitigation: When removal of soluble salts is necessary, the requirements are included in the surface preparation section because the criteria must be achieved prior to coating application. Additional surface preparation and cleaning may be required. Use of proprietary products may be used when salt removal is difficult and salt levels are high. The locations and frequency of testing are important. In the writer’s opinion, the most important surfaces to test are those that have already exhibited corrosion and are the most difficult to prepare. Random testing should also be used to demonstrate absence of soluble salts.

Coated Surfaces: Coated surfaces must be cleaned using low-pressure water during the pre-cleaning process. However, additional cleaning may be appropriate if the surfaces have become contaminated from other operations, such as removal of blistered, cracked or delaminating coating. Glossy and slick surfaces may require scarification to aid in bonding of the overcoat. If SSPC-SP 7, “Brush-Off Blast Cleaning” is warranted, the abrasives should be fine and the nozzle pressure should be kept low. When thick coatings are present and brittle, SSPC-SP 7 may damage otherwise sound coating. Use of abrasive blast-cleaning, even brush-off blast-cleaning, can significantly add to time and cost for an overcoating project and should be carefully considered.

Coating Application Ambient Conditions: Relative humidity (RH); the temperatures of the air, surface and material (Ta, Ts, and Tm); and the dew point (DP) are all to be considered before performing coating application. Minimum and maximum values can be established by the specification. Alternatively, requiring that the ambient conditions conform to the manufacturer’s published requirements is certainly appropriate. This can also avoid confusion or “push-back” by applicators when the manufacturer has more liberal limits than the specification. Yet, if there are sound reasons for being more restrictive, then by all means specify the limits appropriate to the situation.

Mixing and Thinning: Complying with the manufacturer’s instructions is recommended. Thinning may be prohibited, but when permitted, it should comply with the manufacturer’s recommended products for the temperatures and/or VOC limits of the workplace. Induction time and pot life should be considered based on the ambient conditions and the quantities to be mixed.

Spot Application: When application begins, the surface to be coated should meet the surface preparation standard required by the specification. The coatings selected for spot application should lend themselves to brush and roller application and should be applied so that the film extends onto the feathered area of the intact coating. These are typically surface-tolerant coatings such as epoxy-mastic or moisture-cured urethane, among others. Penetrating epoxy sealer (100%-solids) can be used as a first coat where pack rust, crevice corrosion or difficult-to-remove corrosion is present and the spot primer has been applied over it. Stripe coating should be performed with the spot primer and/or tie coat when used. A stripe coat of the finish coat may also be applied, particularly where edge corrosion exists.

Overcoat Application: Following adequate cure of the applied spot coat, a full coat is applied. This may include a tie coat and finish coat or a finish coat compatible with the existing coating resin. Test patches will help to determine which overcoating systems perform better than others. The products used should provide the desired finish (gloss, semi-gloss) and color(s). Brush and roller or spray application can be used depending on surface area, location and how protection of other surfaces from splatter, spits and overspray will be accomplished. It was mentioned earlier that spray application may be prohibited by some owners or circumstances. It is worth noting here that a full overcoat does not necessarily mean to all surfaces. Non-visible surfaces (under deck beams and diaphragms) may not need an additional coat when exposed steel is being overcoated. This is particularly true when the major factor in maintenance painting is aesthetics.

Repairs and Deficient Coating A process for addressing coating repairs from mechanical damage and defects due to preparation or application should be established. In particular, edges of the old coating system may lift when overcoated if thick and not feathered, or if not tightly adhered. These require correction which may be as simple as re-applying the spot coat and/or overcoat, or in the worst cases, removal, preparation and re-application of the spot primer and overcoat.

Quality Control: Quality control must be exercised to achieve a completed project compliant with the specifications, including referenced standards and performance criteria when tested or examined by the methods called for in the specification. We teach that quality control is the responsibility of the contractor. It is; however, the contractor should realize that control over quality is actually in the hands of the craftsmen doing the work (be it surface preparation, mixing or applying the paint) rather than, or in addition to, the foremen and superintendents at the project. The owner and engineer are obligated to clearly state the performance required, the criteria and the manner by which it will be measured. A requirement to submit quality control reports on a regular basis should also be included.

Demobilization: Demobilization is just as much a part of the project as mobilization. It has been included as part of the overcoating specification as a place to house requirements for leaving the site. Requirements typically include cleaning the work and laydown areas, collection and proper disposal of all wastes (including wastewater from low-pressure water cleaning, if applicable) and ensuring no reusable materials leave the site with contamination when toxic materials are involved.

Conclusion: The specification guidance provided herein cannot address every situation or circumstance; no static document can. The complexity associated with what, on the surface, may appear to be a simple spot-and-overcoat solution, has also hopefully been conveyed. Overcoating specifications and methods that are tried-and-true are the progeny of the mistakes, errors, screw-ups and mayhem that came before. Future overcoating specifications will be superior to the ones in use today simply because over time we will notice the scars, warts and shortcomings.

By Richard A. Burgess

KTA-Tator, Inc., Series Editor

1 thought on “Overcoating: Maintenance or Mayhem?”

  1. C & T Paint and Coatings LLC
    What a good article
    I’ve been an independent uncertified Technical Service Rep./ coating Inspector for more then 30 years with thousands of tech service jobs and reports under my belt, and to put it nicely “you hit the nail on the head” in a “Readers Digest” sort of way. As you well understand there’s a lot more to doing a job from start to finish.
    From the first meeting with the perspective buyer of the coating, or the owner of the equipment and the applicator to the last coating inspection and the delivery of the coating report to the owners of the equipment and last, and the most important thing, My signature at the bottom of the report.
    Thank you for the article.

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