SSPC-PA 2 Coating Thickness Acceptance
SSPC-PA 2 is the AMPP standard procedure for deciding whether a coating's dry film thickness conforms to specification. It defines gauge readings, spot measurements, area sampling and the limits that apply.
SSPC-PA 2, “Procedure for Determining Conformance to Dry Coating Thickness Requirements,” is the North American standard that tells inspectors how many thickness readings to take on a coated structure, how to average them, and how to decide whether the coating conforms to the specified minimum and maximum dry film thickness (DFT). It is published by AMPP, the organization formed from SSPC and NACE (see AMPP, SSPC and NACE). The standard works together with ASTM D7091 and similar methods that describe how to use the gauges themselves.
What SSPC-PA 2 covers
PA 2 does not explain how to operate a gauge. That is covered by dry film thickness measurement methods and by coating thickness gauge calibration, verification and adjustment. PA 2 answers different questions:
- How many measurements are needed for a given area?
- How are individual readings combined?
- How far below or above the specified thickness may a single reading or spot be?
- What should happen when an area does not conform?
Key terms
- Gauge reading. A single instrument value at one point.
- Spot measurement. The average of at least three gauge readings taken within a small circle, typically about 4 cm (1.5 in) in diameter. Readings that are unusually high or low may be discarded and repeated within the spot.
- Area. A defined portion of the structure. For areas up to about 9.3 m² (100 ft²), five spot measurements are taken.
- Restriction level. The limit that applies to how far individual spot measurements may deviate from the specified range.
Sampling plan
The standard sets the number of areas to measure based on total surface area:
- Up to about 28 m² (300 ft²): measure each 9.3 m² (100 ft²) area.
- Up to about 93 m² (1,000 ft²): select three random 9.3 m² areas and measure them.
- Larger surfaces: measure three areas in the first 93 m² and one additional area in each further 93 m², or as the specification states.
These numbers are summarized here for orientation. Always use the current edition of the standard, because sampling requirements and wording have changed between editions.
Acceptance rules
Area average
The average of the spot measurements in an area must fall within the specified minimum and maximum DFT.
Spot limits and restriction levels
Individual spot measurements may fall somewhat outside the specified range. In the default case, a spot may be as low as 80% of the specified minimum or as high as 120% of the specified maximum. This is the widely known 80/120 rule. Gauge readings are not limited individually.
PA 2 also defines more restrictive levels for critical service, such as immersion linings or coatings where the manufacturer sets firm upper limits. These levels narrow or remove the allowance. The specification should state which level applies; if it does not, the default applies.
Manufacturer limits
If the product data sheet sets absolute maximum thickness for reasons such as cracking, solvent entrapment or cure problems, the specifier can adopt a restriction level that respects those limits.
Measuring on complex steel
Later editions of PA 2 include appendices on:
- Beams and structural shapes, describing where to take readings on flanges and webs.
- Small parts and test panels.
- Coatings on non-ferrous metals.
- Measurement on test panels for multiple coats.
- Coatings over galvanizing or other metallic layers.
Edges and welds are often measured separately, because thickness naturally falls at edges. See stripe coating and edge corrosion.
Handling nonconforming areas
When an area fails the average or spot limits:
- Map the extent. Take additional spot measurements, often at roughly 1.5 m (5 ft) intervals in each direction, until the boundaries of the low or high area are found.
- Correct low areas by applying additional coating within the recoat window, after cleaning, and remeasure.
- Evaluate high areas against the manufacturer’s maximum. Some products tolerate extra thickness; others may need removal and reapplication.
- Document the corrective action in daily inspection reports.
Multi-coat systems
For multi-coat systems, thickness of each coat can be found by measuring after each coat and subtracting, or by measuring a test panel coated alongside the structure. Destructive methods such as a paint inspection gauge (Tooke gauge) or microscopic cross-sections can measure individual layers. Total system DFT is commonly checked after the final coat.
Common errors
- Measuring before the coating is hard enough, leaving indentations.
- Failing to correct for the base metal reading on blasted steel, as gauge methods require.
- Treating a single low gauge reading as a nonconformance, when PA 2 evaluates spot averages.
- Not stating the restriction level in the specification.
- Mixing units or confusing mils and microns (see mils and microns).
Frequently asked questions
Does PA 2 apply to concrete?
No. PA 2 applies to coatings on metal substrates measured with magnetic or eddy-current gauges. Thickness on concrete is usually checked with ultrasonic gauges, wet film checks or destructive methods, following SSPC-PA 9 or the specification.
Is the 80/120 rule universal?
No. It is the default level in PA 2. Specifications can choose stricter levels, and other standards such as ISO 19840 use different rules.
Who decides the restriction level?
The specifier, ideally with input from the coating manufacturer.
Related entries
- Dry film thickness measurement
- Coating thickness gauge calibration, verification and adjustment
- Wet film thickness measurement
- Coating inspection checklist
- Mils and microns (new)
- ISO 8501-1 visual cleanliness grades (new)
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
