Understanding SSPC-PA 2

Dry film thickness is one of the most discussed — and most misunderstood — aspects of coating inspection. Most inspectors can operate a thickness gage, but far fewer fully understand how SSPC-PA 2 expects measurements to be collected, grouped, and evaluated. When PA 2 is misunderstood, inspections can become either excessive or insufficient. Coatings may pass when they shouldn’t, or fail when they are perfectly acceptable. The goal of this article is to explain how PA 2 actually works, where it truly applies, and how inspectors can use it correctly in both pipeline and coating‑mill environments.

PA 2 does not rely on random gage pulls across a surface. It follows a layered structure. A gage reading is a single instrument measurement. A spot measurement is created by taking at least three gage readings within a small circular area and averaging them. An area measurement is the average of five spot measurements taken within approximately 100 square feet of coated surface. Every evaluated area therefore requires five spot measurements, at least three gage readings per spot, and a minimum of fifteen individual readings. Acceptance is not based on isolated pulls — it is based on averaged data.

An individual gage reading does not determine compliance. A single reading may fall below 80 percent of the specified minimum or above 120 percent of the specified maximum and still not represent a failure. Individual readings are simply data points used to establish a spot measurement. The first meaningful value is the spot measurement — the average of at least three readings taken within a small area. Five spot measurements are then averaged to create the area measurement, which ultimately determines compliance. Individual gage readings may fall outside the 80/20 tolerance, spot measurements are expected to remain within controlled limits, and the average of the five spot measurements must fall within the specified coating thickness range. PA 2 evaluates coatings statistically — not by individual probe pulls.

The commonly referenced 80/20 concept reflects a limited allowance for normal variation at the spot measurement level, while the overall area must still meet the specified coating thickness range. Coatings are applied in real‑world conditions, and some localized variation is unavoidable. PA 2 accounts for that reality without reducing performance requirements. This layered approach ensures minor localized variation does not automatically trigger rejection, systemic under‑application cannot be masked by isolated heavy readings, and the coating system as a whole achieves the intended protective thickness.

PA 2 uses statistical sampling based on total coated surface area. Less than 300 square feet — each portion is evaluated. 300 to 1,000 square feet — three representative areas are measured. Greater than 1,000 square feet — three areas in the first 1,000 square feet plus one additional area for each additional 1,000 square feet or portion thereof. For example, a 12,500‑square‑foot coating application requires fifteen evaluated areas, resulting in seventy‑five spot measurements and at least two hundred twenty‑five individual gage readings. This structured approach produces representative data without unnecessary over‑inspection.

If the area average falls outside specification, or spot measurements exceed allowable limits, additional measurements are taken outward from the non‑conforming location to determine the extent of the condition. This defines whether the issue is localized or widespread and guides repair boundaries.

Before any measurement has value, the gage must be verified. PA 2 requires thickness gages to be checked at the beginning and end of each shift in accordance with ASTM D7091. On high‑volume projects, verification during the shift is common practice. If a gage is dropped or readings appear inconsistent, accuracy must be confirmed immediately. Verification is performed using certified coated standards or shims. Skipping this step undermines inspection credibility and may invalidate collected data.

In many pipeline field projects, coatings are applied to relatively small areas such as field joints, repairs, and tie‑ins, where full statistical PA 2 sampling may not be practical. PA 2 becomes most applicable in controlled environments such as coating mills, where large coated surfaces require statistical verification. For example, a 40‑foot joint of 30‑inch pipe has a coated surface of roughly 314 square feet. Because this exceeds 300 square feet, three sampling areas are required, resulting in fifteen spot measurements and at least forty‑five individual gage readings for that single joint.

As coating inspection expanded beyond flat steel into pipe mills and fabrication environments, additional guidance became necessary for curved surfaces. Appendix 7 was introduced to help apply PA 2 more effectively to coated pipe without changing its statistical structure.

When multiple pipe sections are staged together on a rack or cart, they can be treated as a single inspection unit. The total coated surface is calculated using pipe length, diameter, and quantity. For example, ten pipe sections that are 4 feet long and 9 inches in diameter equal roughly 94 square feet of coated surface — consistent with a single PA 2 inspection area. Because the combined area is less than 100 square feet, five spot measurements are required, consistent with a single PA 2 inspection area (minimum three gage readings per spot). If the total coated surface exceeds approximately 100 square feet, additional spot measurements may be required using a frequency factor. Frequency Factor 2 results in 10 spots, Factor 3 results in 15 spots, Factor 4 results in 20 spots, and Factor 5 results in 25 spots. This allows sampling intensity to increase proportionally with coated surface area.

When pipe spools are measured individually, spot measurements are taken circumferentially and spaced along the pipe length. Up to 12‑inch diameter requires four evenly spaced circumferential spots, 14–24 inch diameter requires six, and greater than 24‑inch diameter requires eight. These circumferential bands are typically repeated at 10‑foot intervals along the pipe, and for spools less than 10 feet in length, three circumferential bands are commonly evaluated to ensure representative coverage. This method maintains statistical intent while adapting to cylindrical geometry.

When applied correctly, PA 2 ensures inspection effort matches the scale of coating work while still producing defensible, representative data. It helps prevent thin coating from being buried, avoids unnecessary rework, and aligns inspection practice with real coating performance. PA 2 is not just a thickness requirement — it is a sampling methodology. Understanding that distinction is what separates routine measurement from true coating inspection.

Established in 2011, Roberts Corrosion Services, LLC delivers comprehensive, turn-key cathodic protection and corrosion control solutions nationwide. Our end-to-end expertise encompasses design and inspection, installation and repair, surveys and remedial work. We provide drilling services for deep anode installations and a full laboratory for analysis of samples and corrosion coupons, as well as custom CP Rectifier manufacturing.

While our initial focus was on the Appalachian Basin area, we complete field work all over the US. We are a licensed contractor in many states and can complete a wide range of services.

Our biggest strength is in our flexibility for our clients. Solutions and Results.

Let us know how we can help.

Website

LinkedIn

Location: 39.251882, -81.047440

(304) 869-4007