A surface can be clean to SA 2.5 and still fail to hold paint.

It happens more often than it should, and the cause is almost always the same: the preparation grade was specified and the profile was forgotten.

What the profile actually is

Every particle that strikes the surface leaves a mark. Millions of overlapping marks form a relief — peaks and valleys, at the scale of microns.

The coating does not sit glued to the metal. It anchors into that relief. The real contact area is far larger than the apparent one, and the layer grips mechanically into every valley.

Hence the established name: anchor profile.

What goes wrong at each end

Too smooth. There is nothing to grip. A coating applied to a clean but polished surface lifts off in sheets — often months later, at the first thermal cycle or the first moisture that reaches the interface.

Too deep. The peaks of the relief pierce the film thickness. Every peak that breaks the surface is a point where corrosion reaches bare metal directly. The coating looks fine and fails in spots.

Between the two extremes there is a range, and the coating system sets that range, not the blasting. The paint manufacturer specifies the required profile; blasting is the means of achieving it.

What influences the profile

Abrasive type. Angular grit cuts and leaves a sharp, deep profile. Round shot peens and leaves a wavier, shallower profile at the same energy.

Size. The most direct factor. Larger abrasive, deeper profile.

Impact velocity. Wheel speed or air pressure. More energy, deeper marks.

The condition of the operating mix. This is where control is most often lost, and most quietly.

Why the profile drifts on its own

In a recirculating installation the abrasive wears. Angular particles round off, shot loses diameter.

A mix that gave the specified profile at commissioning gives something else after a few hundred operating hours — usually shallower. Nothing changed in the machine settings. The operator works the same way, the machine runs the same way, and the profile has moved.

Which is why correctly topping up the mix is not a consumables question, it is a quality question. A mix held in balance gives the same profile this month as last month.

How it is checked

Three methods, in order of accessibility:

The visual-tactile comparator. A reference plate with zones of known roughness. You compare by sight and touch. Fast, subjective, adequate for routine control.

Replica tape. A special film is pressed onto the surface, takes an impression of the relief, and the thickness is measured with a micrometer. Cheap, repeatable, and it leaves a physical record you can attach to the file.

The profilometer. Instrumental measurement with numerical parameters. Used when the specification asks for values, not judgements.

For most industrial painting work, replica tape is the sensible compromise: it costs little and it produces a record.

What to specify, in practice

A serious specification carries both:

The preparation grade — to ISO 8501-1, assessed visually. The profile — a range, not a single value, set by the coating system.

If you receive a specification that asks for only one of them, ask about the other before you blast. It is a five-minute question that prevents several days of rework.

Surface profile: why paint peels off a perfectly clean surface