Surface preparation: why it makes or breaks a coating

When a coating fails early — flaking away in sheets, blistering, or letting rust creep back through — the temptation is to blame the paint. Almost always, the real cause lies further back, in the hours before the first coat ever went on. Surface preparation is the quiet, unglamorous stage where the success or failure of a coating system is decided. Get it right and an ordinary scheme can last for decades; get it wrong and the finest product on the market will let you down.

Why preparation decides adhesion

A coating can only be as good as the surface it sticks to. Paint protects steel by forming a continuous, tightly bonded film that keeps water, oxygen and salts away from the metal. That bond depends entirely on the condition of the substrate underneath. If the surface is contaminated or too smooth, the film has nothing to grip and nothing clean to bond with.

The result shows up as early failure: flaking and peeling where adhesion never formed, blistering where moisture collects beneath the film, and under-film corrosion that spreads unseen until the coating lifts. None of these are product faults. They are preparation faults, and they are almost entirely preventable.

Cleanliness comes first

Before anything else, the surface has to be genuinely clean. Oil and grease must be removed — a coating will not bond through a greasy film, and solvents smeared around carelessly only spread the contamination. Dust, dirt and old, loose or flaking coating all have to go, because a new film is only ever as sound as the layer beneath it.

The most dangerous contaminants are the ones you cannot see: soluble salts. Chlorides and sulphates left on steel — from seawater, de-icing salt or industrial atmospheres — are hygroscopic, meaning they draw moisture through the coating film. Water is pulled toward the trapped salt, pressure builds, and the film lifts into osmotic blisters. Because salts survive blasting if they are not washed away, cleanliness has to be verified, not assumed. Fresh-water washing and salt testing are cheap insurance against a very expensive failure.

Profile and standards

Cleaning removes the bad; profiling creates the good. A coating grips best on a surface with the right roughness — an anchor pattern of tiny peaks and valleys that dramatically increases the bonding area and gives the film something to key into. Too smooth and adhesion suffers; too coarse and peaks may protrude through thin films.

On steel, the benchmark method is abrasive blasting to a defined cleanliness standard. The widely used Sa 2½ ("very thorough blast-cleaning") calls for a surface free of visible oil, scale and rust, with only light shadowing permitted — the practical target for most protective schemes. The standard defines both how clean the steel must be and, alongside it, the profile the abrasive should create. Where blasting is not possible — in occupied buildings, at height, or on maintenance touch-ups — hand and power-tool preparation is used to a lower but still specified standard. It is a fallback, not an equal, and the coating chosen has to suit it.

Common mistakes

Most preparation failures come from a short list of familiar shortcuts:

• Coating over rust or mill scale — the loose layer eventually detaches and takes the coating with it.
• The wrong profile — too shallow to grip, or so coarse that peaks pierce the film.
• Painting in the wrong climate — applying near or below the dew point, or in high humidity, lets an invisible film of moisture form on the steel and ruins adhesion.
• Exceeding recoat windows — leaving too long between coats can stop layers from bonding to each other, so the system delaminates from within.

Where Marisell fits

Good preparation is not guesswork — it is a specification. As a main Hempel dealer, we set the preparation standard and the full coating scheme together, so the surface, the profile and the product are matched from the start. We also provide FROSIO-certified inspection to confirm that cleanliness, profile and climate conditions are right before and during application, and documented afterwards.

Whether the job is heavy protective & industrial steelwork or a maintenance repaint, the principle is the same: the coating is only ever as good as what lies beneath it. Tell us about your project and we'll help you get the foundation right — contact us to start.