A coating holds not because of the paint, but because of what it was applied to. Among protective coating failures, surface preparation is cause number one by a wide margin. And it is the first thing cut when a budget has to be met: it is invisible in the finished work and does not look like a saving to whoever signs the job off. Below: what actually happens during preparation, and where no ground can be given.

Why steel has to be cleaned at all

New steel carries mill scale — a dense layer of oxides formed during hot rolling. It looks like a sound dark crust, and the temptation to paint straight over it is strong. But mill scale is electrochemically nobler than steel: wherever it detaches — and it will — a galvanic couple forms beneath it, and corrosion proceeds rapidly and locally.

To the scale are added rust, shop primer of unknown origin, oil, preservation grease, dust and salts. Each of these interferes with adhesion in its own way. The purpose of preparation is to remove all of it and create a surface the coating has something to hold on to.

Cleanliness grades: what the Sa designations mean

Cleanliness grades are defined by ISO 8501-1 and designated by letters: Sa — abrasive blast cleaning, St — hand and power tool cleaning (wire brushes, grinders).

Sa 1 — light blast cleaning. Loose scale and loose rust removed. Not suitable for protective systems.
Sa 2 — thorough blast cleaning. Most contamination removed; residual traces as stains and streaks are permitted.
Sa 2½ — very thorough blast cleaning. Contamination removed; only faint traces as spots or stripes remain. The working standard for most industrial facilities.
Sa 3 — blast cleaning to visually clean steel. Uniform metallic appearance, no traces. Used for immersion and highly critical structures.

The difference between Sa 2 and Sa 2½ looks immaterial in a photograph. In coating life it is material: residual stains are points where the coating never bonded to the steel, and that is where underfilm breakdown begins.

★ Cleanliness grade is a specification item, not a preference. If a contract says “blast cleaning” without stating a grade to ISO 8501-1, the contractor has formally performed the work whatever the result.

Surface profile: cleanliness is only half of it

The second parameter, forgotten more often than the first, is roughness. Abrasive does more than knock off contamination — it creates the micro-relief that the coating grips mechanically.

Profile is specified by ISO 8503 and divided into grades — fine, medium and coarse. Both deviations do equal harm:

Too smooth — the coating has nothing to key into and adhesion drops.
Too coarse — profile peaks stand proud, and coating thickness over them is far below the average. Those peaks will be the first corrosion points, even though the gauge reads within specification.

The required profile is stated by the coating manufacturer in the data sheet, and it is tied to the specified thickness: the thicker the coat, the coarser the permissible profile.

What cannot be seen: salts and dust

A surface can look immaculately clean and still be unfit to coat.

Soluble salts — chlorides above all — remain on the steel after cleaning and act as a pump beneath the coating: they draw moisture through the film, generate osmotic pressure and blister the coating from within. Outwardly this appears as blisters on a new coating some months after handover. They are measured by a soluble salt test (the Bresle method to ISO 8502), carried out on site in minutes.

Dust always remains after blast cleaning. If it is not removed, the coating is applied to dust rather than to steel. Dust quantity is also specified — by ISO 8502-3. Blowing down with compressed air must be done through a water and oil separator, otherwise compressor oil arrives on the surface along with the dust, making matters worse.

Dew point and the time window

Two rules broken more often than any others, because they concern the programme rather than workmanship.

The steel temperature must be at least 3 °C above the dew point. Otherwise moisture condenses on the surface — invisible, but enough to ruin adhesion. Dew point is calculated from air temperature and humidity measured on site, not taken from a forecast.

Cleaned steel must be coated the same day. Bare steel begins to oxidise immediately: first an invisible bloom, then visible ginger rust — flash rusting. In dry weather the margin is longer; in damp weather it is a matter of hours. Blast cleaning on Friday and painting on Monday means paying for the cleaning twice.

What is used for cleaning, and why it is not a detail

The abrasive affects the outcome no less than the equipment.

Damp abrasive leaves moisture and adhering particles on the surface. Abrasive must be dry — a handful in the palm is test enough.
Recycled abrasive works if it is screened and cleaned. Without that, oil, salts and fractured particles from already-cleaned surfaces return into the blast stream — contamination is simply carried from one area to another.
Silica sand fractures into fine crystalline silica dust, the inhalation of which causes silicosis — a severe and incurable lung disease. In many countries its use in blast cleaning is restricted or prohibited, and work is carried out with copper slag, garnet or steel grit. This is the case where saving on a material is paid for with workers’ health.

Six places where corners are cut, and what follows

Reducing the cleanliness grade from Sa 2½ to Sa 2. The saving shows in the budget immediately; the difference in outcome shows in two or three years.
Skipping edge and weld preparation. Sharp edges, weld spatter and undercuts are left because dressing them is manual work and time. Coating on an edge is always thinner, and breakdown starts there rather than on the flat.
Economising on dust removal. The cheapest operation in the whole cycle and one of the most effective.
Omitting the salt test. The kit costs money, the test takes time, and the client cannot see the result. Yet on sites near chemical plants it is salts that most often kill the coating.
Stretching the interval between cleaning and coating. Usually not from economy but from poor planning — the outcome is the same.
Dropping the stripe coat. A separate brush pass over edges, welds and bolted connections before the main coats looks like a redundant operation. It is the only way to achieve the specified thickness on an edge.

How all of this is verified

Surface preparation is the one stage that cannot be verified afterwards. Once the first primer coat is on, nothing beneath it can be seen. Inspection is therefore carried out before coating, and recorded:

cleanliness grade — visually against the ISO 8501-1 reference photographs;
profile — by comparator or profile gauge;
salts — by soluble salt test;
dust — to ISO 8502-3;
steel and air temperature, humidity and dew point — by instrument on site, entered in the log.

For a trained inspector all of this takes minutes per work area. The absence of these records from the handover documentation is grounds for not accepting the work, however good the coating looks.

In brief

Surface preparation is not a stage before painting — it is half of the coating itself. Economising here does not reduce the cost of the work; it defers that cost with a heavy multiplier: instead of cleaning clean steel at ground level, the whole cycle has to be repeated on an erected structure, with scaffolding and a production shutdown.

Four items belong in every specification: cleanliness grade to ISO 8501-1, required profile, a mandatory salt test, and dew point monitoring with records. If any one of them is missing, there is nothing on which to accept the work.

The next article in this series examines what our ultraviolet does to coatings, and why a system that performs well in temperate regions may not reach half its design life here.

Surface preparation for protective coatings: where you must not cut corners