Concept · Surface preparation
Surface profile (anchor pattern)
Surface profile is the roughness blasting leaves on steel. Why coatings need it and how comparators, replica tape and depth gauges measure it.
Abrasive blasting cuts small peaks and valleys into steel. Their height, from valley floor to peak, is the surface profile or anchor pattern. It gives the coating a larger, rougher surface to grip.
The why
A smooth plate gives the coating little to hold. A blasted plate has more surface area. The liquid coat flows into the valleys and hardens around the peaks. The dry film then locks onto the steel like a key in a lock.
Both too little and too much profile cause failures. Too little profile weakens the key. Too much profile leaves tall peaks. A tall peak can stand close to the film surface or through it, and it rusts early.
The profile also affects the thickness reading. The inspector takes a base metal reading on bare blasted steel and subtracts it from the coating readings. Read dry film thickness measurement for the method.
The abrasive sets the shape. Angular grit cuts sharp peaks. Round shot leaves dimples (ISO 8503-1). A larger particle hits harder and cuts deeper, but fewer particles hit each unit of area.
What the specification asks for
| Source | Profile asked for |
|---|---|
| IMO PSPC ballast tanks | 30–75 µm (1.2–3 mils) with Sa 2½ |
| ISO 12944-9, offshore systems | Medium (G) grade (ISO 8503-1) with Sa 2½ at least |
| SSPC-SP 11 and SP 15 | At least 25 µm (1 mil) |
| SSPC-SP 16, non-ferrous metals | At least 19 µm (0.75 mil) |
SSPC-SP 10/NACE No. 2 sets no profile depth. The abrasive must give the profile that the specification or the data sheet asks for. SP 3 power tool cleaning sets no profile requirement. Waterjetting does not create profile. It exposes the profile that is already there (see waterjetting).
How it is measured
- Comparators (ISO 8503-1 and -2, ASTM D4417 Method A): a reference surface compared by eye, with a lens of ×7 at most, or by touch. The procedure applies only to Sa 2½ and Sa 3 surfaces.
- Depth micrometer (ASTM D4417 Method B): a flat base rests on the peaks and a pointed probe reads the valley depth.
- Replica tape (ASTM D4417 Method C, ISO 8503-5): foam compressed onto the steel and read with a spring micrometer. Subtract the 50.8 µm (2 mil) film that does not compress.
- Stylus instruments (ISO 8503-4, ASTM D4417 Method D): a profile trace with roughness values. Method D replaced ASTM D7127 in 2021.
A comparator gives a grade, not a number. Each comparator has four segments. The grade limits are:
| Grade | Grit (G) | Shot (S) |
|---|---|---|
| Fine | 25 to under 60 µm | 25 to under 40 µm |
| Medium | 60 to under 100 µm | 40 to under 70 µm |
| Coarse | 100 to under 150 µm | 70 to under 100 µm |
For a mix of shot and grit, use the G comparator. Check a comparator in frequent use against an unused one every three months.
In the field: replica tape
- Choose the tape grade for the expected profile. Coarse tape covers 20–64 µm (0.8–2.5 mils). X-Coarse covers 38–115 µm (1.5–4.5 mils).
- Burnish the tape onto the blasted steel until the foam shows an even impression.
- Peel the tape off and place it in the spring micrometer. Read it twice. The mean of two readings is one measurement.
- Subtract the 50.8 µm (2 mil) film, or set the gauge zero to −50.8 µm.
- Compare the result with the specified range. Make as many measurements as the parties agreed.
Worked example
The specification asks for 50–85 µm. Two tape readings are 3.9 and 4.3 mils. The mean is 4.1 mils. Subtract the 2 mil film to get 2.1 mils, which is 53 µm. The profile is in range.
Check: 4.1 − 2.0 = 2.1 mils, and 2.1 × 25.4 = 53.3 µm. Without the subtraction, 4.1 mils is 104 µm, and the surface would fail on a false reading.
What the inspector records
Record the method and the standard, the number of measurements and their locations, the result for each, and the instrument serial number. Measure again if the steel waits or is handled before coating, because rubbing can flatten the peaks.
Exam traps
- Trap: the micrometer reading on replica tape is the profile. It includes the 50.8 µm (2 mil) film, so subtract it unless the gauge was zeroed for it.
- Trap: a comparator suits any prepared surface. ISO 8503-2 limits it to Sa 2½ and Sa 3.
- Trap: ASTM D4417 has three methods. It has four since 2021.
- Trap: waterjetting cuts a new profile. It only exposes the old one.
- Trap: a deeper profile is always better. The specification gives a maximum as well as a minimum.
- Trap: replica tape and a depth micrometer give the same number. They differ in a systematic way, so do not swap them.
- Trap: SP 11 and SP 16 ask for the same minimum. SP 11 asks for 25 µm (1 mil), and SP 16 asks for 19 µm (0.75 mil).
How the exams treat it
The CIP Level 1 practical has an anchor pattern station worth 20 points, with comparators and replica tape. The CIP Level 1 guide names ASTM D4417. One FROSIO training body lists roughness measurement with ISO comparators among its practical stations. Practise the cleanliness side on surface cleanliness grades.
Use it: Practical station: replica tape · Surface preparation grades
193 practice items in the members area mention this topic (CIP 1: 87 · CIP 2: 75 · FROSIO written: 31).
Related
Try it in an exam-style question. The free readiness check shows the question style, explains every answer and gives you a dated study plan.
Take the free readiness checkSee the tracksAbout this page
Sources: ISO 8503-1:2012 · ISO 8503-2:2012 · ISO 8503-4:2012 · ISO 8503-5:2017 · ASTM D4417-21 · SSPC-SP 10/NACE No. 2-2024 · SSPC-SP 11 (2020) · ISO 12944-9:2018 · IMO MSC.215(82).
Also published openly: Elcometer 125 surface comparators.
Written from the published standards and coating practice, in our own words, and checked on 2026-10-04. Found an error? It goes in the corrections log. CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.