Surface profile and prep standards

Two specifications, not one

Surface preparation is judged on two independent things. Confusing them is the most common reason a coating fails early.

Cleanliness is how much rust, mill scale, old coating and contamination has been removed. It is graded visually against photographic standards.

Profile — also called anchor pattern or surface roughness — is the microscopic peak-to-valley texture the blasting leaves behind. It is measured in microns and it is what gives the coating something to key into.

You can have one without the other. Chemical stripping produces a clean surface with no profile. Blasting with the wrong media grade can produce a profile that is either too shallow for the coating to grip, or so deep the peaks stand proud of the paint film and rust through it.

Cleanliness grades

Two systems dominate, and they map onto each other closely. ISO 8501-1 is the international standard, referenced by AS 1627.9 in Australia. SSPC/NACE is the American system you will see on imported datasheets.

Sa 2½ is the workhorse. It is what most industrial and marine coating systems specify, it is achievable with normal equipment, and it is inspectable. Sa 3 costs substantially more for a marginal gain and is reserved for immersion service and tank linings.

What Sa 2½ looks like

Uniform grey-white metal, with faint darker shadows where pitting or old scale sat. It is not mirror clean and it is not meant to be. Anything more than light shading over 5% of the area fails.

Inspection is visual, against the photographic plates in ISO 8501-1. The comparison must be made under adequate light, and the grade is judged over each square metre — not averaged across the whole job. On specified work we photograph representative areas as a record.

Anchor profile

Profile is the peak-to-valley height of the blasted surface, in microns (µm). One micron is a thousandth of a millimetre.

The rule of thumb: profile should be roughly one quarter to one third of the total dry film thickness of the coating system. A 150 µm system wants somewhere near 50 µm of profile.

• Too shallow (under about 25 µm) — nothing for the coating to key into. Adhesion depends on chemical bond alone and the film peels in sheets.
• Correct (typically 40–75 µm for general work) — the coating flows into the valleys and locks mechanically.
• Too deep (over the film thickness) — peaks protrude through the coating. Those points rust first and the failure spreads outward from each one, giving a pinpoint rash of corrosion across the panel.

Profile is controlled by media grade, hardness and pressure. Coarse media at high pressure equals deep profile; fine media at low pressure equals shallow. See the media guide.

How profile is measured

Three practical methods:

• Testex replica tape — compressible foam tape burnished onto the surface, then read with a spring micrometer. Cheap, portable, gives a physical record you can staple to the report. The standard method on site.
• Digital profile gauge — a needle probe reading peak-to-valley directly. Fast for taking many readings.
• Comparator discs — a reference disc held against the surface for visual and tactile comparison. Quick, rough, fine for unspecified work.

On specified jobs we take readings across representative areas and record them. If your project needs documented preparation, say so at quote stage — it changes the process, not just the paperwork.

Reading a coating datasheet

Under "surface preparation" a typical industrial datasheet gives you four things: a cleanliness grade, a profile range, a maximum soluble salt level, and a recoat window. All four are conditions of the warranty.

The recoat window is the one that catches people out. It is the time between blasting and first coat, and it is why sequencing matters more than almost anything else on a job. Blast on a Friday, coat on a Monday, and on a specified job you have already failed — the surface has to be reblasted.

Where a wet process is used, the datasheet may also state a maximum permissible flash rust grade. See flash rust, explained.

Soluble salts

Chlorides and sulphates left on the steel draw moisture straight through the coating film by osmosis, and cause blistering and undercutting no amount of profile will prevent. This is a serious issue in WA marine and coastal work — anything that has sat near the ocean carries chloride contamination.

Blasting alone does not remove soluble salts; it can drive them into the profile. Freshwater washing, sometimes with a salt remover, is the fix. Levels are tested with a Bresle patch and a conductivity meter, and typical specifications allow no more than 20–50 mg/m².

Wet abrasive blasting has a real advantage here — the water flushes salts as it goes, and on chloride-contaminated steel it often produces a cleaner result than dry blasting followed by a wash.

How to write a spec that holds

If you are engaging a blaster for coated work, put these five things in writing:

1. The cleanliness grade, by standard and number — "Sa 2½ to ISO 8501-1", not "clean".
2. The profile range in microns, and the measurement method.
3. The maximum soluble salt level, if the item is coastal, marine or immersion service.
4. The maximum time between blasting and first coat, and who is responsible for coating.
5. Who inspects, and what evidence is required — tape readings, photographs, a signed hold point.

That list turns a vague expectation into something both sides can hit. It also usually reveals that the job needs less preparation than the spec inherited from a previous project demanded, which saves money.