Concept · Surface preparation
Waterjetting and flash rust
The WJ-1 to WJ-4 cleaning grades, why waterjetting exposes but does not create profile, and the light, medium and heavy flash rust grades.
High-pressure waterjetting removes coatings, rust and soluble salts without abrasive. It does not create a new profile. It exposes the profile already on the steel.
The why
Abrasive blasting makes profile because hard particles hit the steel. Each hit dents and cuts the surface. Water carries no abrasive. The jet lifts off what is weaker than the steel, such as paint, rust and scale, and does not cut the steel itself.
The profile after waterjetting is the profile before it. Old coating or rust can hide this profile, and the jet then shows it. Smooth steel stays smooth. Where the specification asks for a profile, a different method must make it. Wet abrasive blasting is one such method, because the abrasive still cuts.
The same water that does not cut steel does dissolve salt. Waterjetting flushes soluble salts out of pits, where dry blasting leaves them. ISO 8501-1 says that wet blast cleaning or water jetting should be used for salts. The water must be clean. Test the result with the Bresle method (soluble salts).
Pressure classes
| Class | Pressure |
|---|---|
| Low-pressure water cleaning | Below 34 MPa (5,000 psi) |
| High-pressure water cleaning | 34–70 MPa (5,000–10,000 psi) |
| High-pressure waterjetting (HP WJ) | 70–210 MPa (10,000–30,000 psi) |
| Ultra-high-pressure waterjetting (UHP WJ) | Above 210 MPa (30,000 psi) |
Waterjetting in the standards starts at 70 MPa. A pump that reads in bar needs a conversion: 10 bar is 1 MPa. A pump at 1,500 bar gives 150 MPa, which is HP WJ. A pump at 2,500 bar gives 250 MPa, which is UHP WJ.
Check: 2,500 ÷ 10 = 250 MPa, and 250 is above 210. UHP is a safety matter too. The jet can inject water under the skin.
Cleaning grades
The joint SSPC/NACE standards name four grades. The ISO 8501-4 standard names four Wa grades, and the close counterparts follow.
| SSPC/NACE grade | Meaning | Residue allowed | ISO 8501-4 |
|---|---|---|---|
| WJ-1 | Clean to bare substrate | None | Wa 3 |
| WJ-2 | Very thorough cleaning | Up to 5 % of each unit area | Wa 2½ |
| WJ-3 | Thorough cleaning | Up to 33 % of each unit area | Wa 2 |
| WJ-4 | Light cleaning | More than WJ-3 | Wa 1 |
The unit area is the same as for blast cleaning, about 76 × 76 mm. The pairs are close counterparts, not identical definitions. ISO 8501-4 also names the starting condition: degraded paint (DC A, DC B and DC C) or degraded shop primer (DP I and DP Z). The reference photographs for waterjetting are SSPC-VIS 4/NACE VIS 7.
Flash rust
Bare steel has no mill scale and no coating to protect it. Wet steel and air make iron oxide quickly, and the rust forms within minutes as the surface dries. Salts left on the steel make it worse, because they keep the surface wet.
| Grade | What the inspector sees |
|---|---|
| Light (L) | The steel shows through. The rust is tight and does not wipe off easily. |
| Medium or moderate (M) | The rust hides the steel. It sticks fairly well and leaves light marks on a lightly wiped cloth. |
| Heavy (H) | The rust hides the steel fully. It is loose and leaves strong marks on a cloth. |
ISO 8501-4 says medium, and the SSPC/NACE standards say moderate. The specification states the highest grade allowed before coating. Some coatings accept light flash rust. Heavy rust must be removed, and the surface is then inspected again.
In the field
- Read the specification for the grade, the highest flash rust grade, the profile and the salt limit.
- Compare the dry steel with the reference photographs of the grade.
- Wipe a clean cloth lightly over the rust to place it in grade L, M or H.
- Test salts with the Bresle method after the wash and the drying.
- Check the steel temperature against the dew point. Steel at or below the dew point gets wet again.
- Check the profile. Do not expect the jet to have changed it.
- Note the time between cleaning and the first coat.
Some jobs add a rust inhibitor to the water or hold the steel in dry air. The specification states what is allowed. Dehumidified air keeps the steel dry and lengthens the time before priming.
What the inspector records
Record the grade and the standard, the flash rust grade, the profile result and the salt result. Add the steel temperature, the dew point and the time of the check.
Exam traps
- Trap: waterjetting cuts a new profile. It exposes the old profile and cuts none.
- Trap: WJ-2 is the bare-substrate grade. WJ-1 is, and WJ-2 allows 5 % residue.
- Trap: waterjetting begins at 34 MPa. Low-pressure water cleaning is below 34 MPa and waterjetting starts at 70 MPa.
- Trap: ISO 8501-4 has three Wa grades. The 2020 edition has four, with Wa 3 to bare substrate.
- Trap: flash rust means that the cleaning failed. It forms on good work as the wet steel dries.
- Trap: a clean look after waterjetting means no salts. Test them.
How the exams treat it
The CIP Level 2 guide names waterjetting. The FROSIO book gives the four Wa grades and the three flash rust names.
40 practice items in the members area mention this topic (CIP 1: 13 · CIP 2: 20 · FROSIO written: 7).
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: SSPC-SP WJ-1 to WJ-4/NACE WJ-1 to WJ-4 (2012) · ISO 8501-4:2020 · ISO 8501-1:2007 · SSPC-VIS 4/NACE VIS 7.
Also published openly: AMPP store: NACE WJ-2/SSPC-SP WJ-2 description.
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.