AWWA D102 Explained: Coating Standards for Steel Water Storage Tanks

Key Takeaways

• AWWA D102 establishes minimum requirements for coating welded steel water storage tanks.
• The standard covers coating materials, coating systems, surface preparation, application, inspection and testing.
• Outside Coating Systems (OCS) and Inside Coating Systems (ICS) are designed for different service environments.
• Proper specification, application and inspection are critical to long-term coating performance.

In This Article

• What Is the AWWA D102 Standard?
• The Basics of AWWA D102: What's Covered?
• Outside Coating System (OCS) vs. Inside Coating System (ICS)
• How Your Specification Should Complement AWWA D102 
• AWWA D102 FAQs
• Protect Steel Water Tanks with Coatings from Sherwin-Williams

What Is the AWWA D102 Standard?

The American Water Works Association (AWWA) D102-24 Coating Steel Water-Storage Tanks standard provides minimum requirements for coating steel water storage tanks, including coating materials, coating systems, surface preparation, application, inspection and testing. It helps define how steel water storage tanks should be coated or recoated for potable water service and applies to coating systems used on both interior and exterior surfaces of steel tanks used for water storage in water supply service.

Interior and exterior tank surfaces face very different conditions. The inside of the tank may be in constant contact with potable water, while the outside is exposed to weather, ultraviolet light, moisture, temperature changes and atmospheric contaminants. AWWA D102 helps address these different exposures through defined coating system categories and project requirements.

The standard is often referenced in project specifications, bid documents and engineering requirements—and in some cases, mandated by local regulations. When it’s included in contract documents, it gives all project stakeholders a shared baseline for the work. Owners know what level of protection is being specified, engineers have a recognized standard to reference and contractors understand the coating and application expectations.

It’s also important to understand what AWWA D102 does not cover. Coating systems for bolted steel tanks are not described under AWWA D102. Those are addressed under ANSI/AWWA D103. For welded steel potable water storage tanks, however, AWWA D102 is a central reference point.

The Basics of AWWA D102: What’s Covered?

Coating performance depends on every step of the process working together. AWWA D102 addresses the full process, from material selection through inspection and testing.

Coating Materials

AWWA D102 addresses the coating materials used for steel water storage tanks. Coating selection should be based on the tank surface, exposure, water service requirements, atmospheric conditions, project goals and manufacturer recommendations.

For interior potable water storage, coating materials must be suitable for immersion service and water contact. Minimum performance standards are included in the coating systems. In many projects, coatings or linings used in contact with potable water must also meet applicable drinking water requirements, such as NSF/ANSI/CAN 61 and NSF/ANSI/CAN 600.

Sherwin-Williams offers potable water storage coatings and linings that are NSF/ANSI 61/600 approved and designed to help protect tanks, piping and valves, including epoxy and polyurethane technologies built for corrosion resistance, chemical resistance and long-term performance in demanding water storage environments.

Coating Systems

Depending on the project, a coating system may include a primer, intermediate coat, finish coat, lining or other system components, but it’s also important to understand what is NOT included in the standard. Stripe coating is, for example, not required by AWWA D102, so if you want a stripe coat, you will need to specify it.

Each layer has a role. For example, a primer may help promote adhesion and corrosion resistance, while the finish coat can provide weathering resistance, color retention and gloss retention on exterior surfaces. Interior lining systems are often designed to create a protective barrier between potable water and the steel substrate.

Because these components work together, product compatibility is key. Substituting one product without reviewing the full system can create adhesion, cure, performance or compliance problems. The system should be selected as a complete assembly.

Surface Preparation

Surface preparation is one of the most important factors in coating performance. Coatings need a clean, properly prepared substrate to bond effectively. Rust, mill scale, oil, grease, soluble salts, moisture, dust and old coating residue can interfere with adhesion and shorten service life.

For new steel tanks, surface preparation focuses on removing mill scale, creating the specified surface profile and preparing welds, edges and details. For existing tanks, preparation may also involve removing failed coatings, evaluating existing systems, addressing corrosion, repairing pitted areas and determining whether remaining coatings are compatible with the new system.

Surface profile is also important. Many protective coatings require a specific anchor pattern to promote mechanical adhesion. Too little profile can lead to poor adhesion, while too much profile can create peaks that are difficult to cover, increasing the risk of corrosion at high points.

Edges, welds, seams, ladders, nozzles and appurtenances need special attention. These areas are more difficult to coat evenly and are often where failures begin. Proper preparation and stripe coating can help improve protection in these vulnerable areas.

Application Requirements

Application conditions play a major role in final coating performance. A coating that looks acceptable during application may still fail if it was applied under poor environmental conditions or outside the manufacturer’s instructions.

Important application factors include substrate temperature, air temperature, relative humidity, dew point, ventilation, mixing, thinning, pot life, induction time, cure time and recoat windows. Applicators must also monitor wet film thickness and dry film thickness to meet the specified system requirements.

Interior tank work can be especially demanding. Applicators may be working in confined spaces with ventilation requirements, access limitations, humidity control concerns and strict return to service timelines. Coatings used for potable water immersion must cure properly before the tank is placed back into service.

Inspection and Testing

Inspection and testing help confirm that the coating work aligns with the project specification and standard requirements. The D102 standard requires verification of surface preparation requirements, wet film thickness and dry film thickness testing along with holiday testing below the maximum water level. Additional testing to consider can include visual inspection, surface profile measurements, environmental readings, holiday detection above the water line and in the roof, cure evaluation and documentation review.

For interior linings, holiday testing is particularly important. A holiday is a pinhole, void, missed area or discontinuity in the coating film. A small defect can expose steel to water and create a pathway for corrosion. Finding and repairing these defects before the tank returns to service can prevent larger problems later.

Documentation should be treated as part of the project, not an afterthought. Inspection reports, batch numbers, environmental logs, film thickness readings, repair logs and cure records all help support compliance, future maintenance, warranty discussions and long-term asset planning.

Outside Coating System (OCS) vs. Inside Coating System (ICS)

AWWA D102 includes coating system categories for exterior and interior tank surfaces. These are commonly discussed as outside coating systems and inside coating systems.

Understanding the difference between OCS and ICS is critical because exterior and interior tank surfaces face different threats. The wrong system in the wrong environment can shorten the life of the coating and create costly problems.

What Are Outside Coating Systems?

Outside coating systems are used on exterior steel tank surfaces. Their main job is to protect the tank from atmospheric corrosion while also maintaining appearance. Over the last couple decades, aesthetics have become as important to tank owners as corrosion protection.

Exterior tanks face sunlight, ultraviolet exposure, rain, snow, ice, wind, humidity, condensation, airborne contaminants and temperature swings. In coastal or industrial environments, exterior tanks may also face salt, chemicals or more aggressive atmospheric conditions.

Because exterior tank coatings are visible, aesthetics may also be an important consideration. Water tanks often serve as landmarks in a community. Color selection, logo placement, striping and long-term appearance can be part of the project scope. A coating system may need to provide corrosion protection while also supporting the visual identity of a municipality, utility, school, industrial site or community.

Exterior systems often include a corrosion resistant primer, an intermediate coat for barrier protection and a finish coat designed for weathering and color retention. The exact system depends on exposure, service expectations, surface preparation, project budget and specification requirements.

What Are Inside Coating Systems?

Inside coating systems are used on the interior surfaces of steel water storage tanks. These systems are often referred to as linings because they protect steel surfaces in immersion service.

The interior environment is very different from the exterior. They must resist corrosion, maintain adhesion in immersion conditions and support water quality requirements.

Additionally, interior linings may be in continuous or intermittent contact with potable water. For potable water service, interior linings are often required to meet NSF/ANSI/CAN 61 and related extraction requirements to help confirm that the coating is appropriate for drinking water contact under defined conditions.

Interior linings must also be applied and cured correctly before the tank is returned to service. If the lining is placed into service too soon, or if cure conditions are poor, the system may not perform as intended. That can affect adhesion, chemical resistance, taste and odor concerns or overall service life.

Sherwin-Williams offers several potable water storage lining technologies designed for steel tanks and related water infrastructure. Some systems are engineered for high build application, edge retention, low temperature cure, rapid return-to-service or compatibility with specific AWWA D102 system categories. All ICS’s are not equivalent in expected service life. ICS 3 is a 100% solids epoxy lining system with a life expectancy of 25+ years and ICS 4 is a 100% solids polyurethane lining system with a life expectancy of 30 years. These systems are the most cost effective from a life cycle perspective and the most sustainable solutions.

Key Differences Between OCS and ICS

Outside coating systems and inside coating systems both protect steel water storage tanks, but they are designed for very different environments. OCS protects the visible exterior from weather and atmospheric corrosion and ICS protects interior steel surfaces in potable water immersion service. Because each system faces different exposure, performance and inspection demands, they are generally not interchangeable.

The key takeaway is that OCS protects the tank from the outside environment while ICS protects the tank from the inside service environment. 

How Your Specification Should Complement AWWA D102

Maintaining compliance with AWWA D102 starts with a clear project scope, accurate specifications, proper system selection and a realistic plan for surface preparation, application, inspection and maintenance. 

Start with Accurate Project Specifications

A successful coating project begins with a strong specification. The specification should reference the correct version of AWWA D102 and define which areas of the tank are included in the work. It may include the shell, roof, floor, interior surfaces, exterior surfaces, ladders, platforms, piping, nozzles, vents, manways, overflow structures and other appurtenances.  

The more clearly the project is defined, the easier it is for bidders, contractors, inspectors and owners to align expectations. 

A strong specification should also document key service conditions that informed coating selection. Is the tank elevated or ground supported? Is it located in a rural, coastal, industrial or high-humidity environment? What is the water chemistry? What is the expected downtime window? Are there return-to-service requirements? Are there special safety, access or scheduling concerns? 

Capturing these details ensures the specified system is applied correctly and that project execution aligns with real-world operating conditions. 

Select Coating Systems Based on Service Conditions

When selecting a coating system, owners and engineers should consider the total cost of ownership, including expected service life, maintenance intervals, repair complexity, downtime, water service disruption and future recoating needs. A coating system that costs more upfront may provide better long-term value if it reduces maintenance, extends service life or shortens return to service time. 

For potable water tanks, product selection should also account for regulatory and project specific requirements. Interior linings must be appropriate for potable water contact and immersion service and exterior systems should be selected based on atmospheric exposure, weathering needs and appearance goals. 

Coating manufacturers can be valuable partners during this stage. At Sherwin-Williams, our team can help you evaluate service conditions, compare coating technologies, review specification requirements and identify suitable systems for water storage applications. 

Prepare Surfaces Properly

Surface preparation is one of the most common areas where coating projects succeed or fail. 

Before coating begins, the steel surface must be cleaned and prepared according to the specification and coating manufacturer requirements. This process may involve abrasive blasting, cleaning, grinding, feathering, edge preparation, removal of old coating, treatment of corrosion and repair of surface defects. 

Monitor Application Conditions

Even with the right coating and proper surface preparation, application conditions can make or break the project. 

Applicators should monitor temperature, humidity, dew point, ventilation and substrate conditions before and during application. Coatings should not be applied when moisture is present on the surface or when environmental conditions fall outside product requirements. 

Mixing and handling also make a difference. Two component coatings must be mixed at the correct ratio. Pot life, induction time, thinning restrictions and application equipment should follow the manufacturer data sheet. Film thickness must be checked throughout the process. 

If the coating is too thin, it may not provide the required protection. If it’s too thick, it may crack, solvent trap, cure improperly or fail to meet product limitations. Proper thickness control supports both performance and compliance. 

Inspect and Document the Work

Inspection should be built into the project schedule. Inspection activities may occur before surface preparation, after surface preparation, during application, between coats, after cure and before return-to-service. Each stage gives the project team a chance to identify and correct problems before they become harder or more expensive to fix. 

Documentation should include surface preparation records, environmental readings, coating batch numbers, product data sheets, application dates, wet film readings, dry film readings, holiday testing results, repair logs, cure times and final inspection reports. 

Good documentation gives owners a reliable record of what was done and helps future teams understand the tank history when maintenance, repair or recoating work is needed. 

Maintain the Tank Over Time

Long-term performance depends on routine inspection and maintenance. This starts at the warranty inspection. A thorough evaluation should be performed prior to the expiration of the specified warranty period. The applicator should be present and equipped to repair any defects that are identified. The majority of any defects will manifest within the first year and correcting the defects at this time before they become worse is a major step toward long term performance. 

Over time, tank owners should look for signs of coating wear or failure, including rust staining, chalking, cracking, blistering, peeling, delamination, mechanical damage, sealant failure, corrosion at edges or welds, roof issues and interior lining defects. 

Even if issues are small, they should be addressed before they spread. A minor exterior coating defect may be repairable during routine maintenance. Left untreated, that same defect can lead to corrosion, underfilm failure and larger rehabilitation costs. 

Interior inspections are also important. Because interior linings are hidden during normal operation, defects may go unnoticed until the tank is taken out of service. Planned inspection cycles help owners identify lining wear, corrosion, sediment accumulation and other concerns before they affect performance. 

AWWA D102: FAQs

What Does AWWA D102 Stand For?

AWWA D102 is the American Water Works Association standard for coating steel water storage tanks. It is commonly used for steel potable water storage tank coating and recoating projects. 

What Does AWWA D102 Cover?

AWWA D102 covers minimum requirements for coating steel water storage tanks, which includes coating materials, coating systems, surface preparation, application, inspection and testing. 

Does AWWA D102 Apply to Both Inside and Outside Tank Surfaces?

Yes. AWWA D102 addresses coating systems for both inside and outside surfaces of steel tanks used for potable water storage in water supply service. 

Does AWWA D102 Apply to Bolted Steel Tanks?

No. Coating systems for bolted steel tanks are addressed under ANSI/AWWA D103, not AWWA D102. 

What Is the Difference Between OCS and ICS?

OCS refers to outside coating systems used on exterior tank surfaces. ICS refers to inside coating systems used on interior tank surfaces, often in potable water immersion service. Exterior systems focus on atmospheric corrosion, weathering and appearance. Interior systems focus on immersion resistance, corrosion protection, cure, lining continuity and potable water suitability. 

Why Is Surface Preparation So Important for AWWA D102 Projects?

Surface preparation affects coating adhesion, corrosion resistance and service life. Contaminants, rust, old coating, moisture or poor surface profile can cause early coating failure, even when the selected coating is high quality. 

Do Potable Water Tank Linings Need NSF/ANSI 61 Approval?

Potable water projects require interior linings to meet NSF/ANSI/CAN 61 and related requirements. Project teams should review local regulations, owner standards and project specifications before selecting a coating or lining system. 

How Often Should Steel Water Storage Tanks Be Inspected? 

Inspection frequency depends on the tank age, coating condition, service environment, water chemistry, regulatory requirements and owner maintenance program. Tanks in aggressive environments or with known coating concerns may need more frequent review. 

Can Existing Tanks Be Recoated Under AWWA D102?

Yes. AWWA D102 addresses coating and recoating of steel tanks used for potable water storage. Existing tanks may require additional evaluation because old coatings, corrosion and surface conditions can affect the recoating approach. 

Protect Steel Water Tanks with Coatings from Sherwin-Williams

At Sherwin-Williams Protective & Marine, we offer high performance coating and lining technologies for water storage environments, backed by technical support for project teams working through specification, product selection, application and maintenance questions. Our potable water storage coatings and linings are built to support corrosion protection, chemical resistance, clean water delivery goals and long-term asset protection. 

If you are planning a steel water storage tank coating, recoating or rehabilitation project, contact us today to discuss your tank conditions, performance goals and coating system options.