What is the difference between Sa 2.5 and St 3 surface preparation?

Sa 2.5 is a blast cleaning standard (near white metal) achieved by shot blasting. It removes all rust, mill scale, and old coatings, and creates a surface profile of Rz 40–70 µm. St 3 is the highest standard achievable by wire brushing — it removes loose rust and paint but cannot remove mill scale and creates no surface profile. Sa 2.5 is required by virtually all industrial coating manufacturers for their warranty to apply.

Does wire brushing remove mill scale?

No. Wire brushing cannot reliably remove mill scale from steel. Mill scale is a hard oxide layer formed during the steel rolling process. It is cathodic to steel, meaning it accelerates corrosion wherever it is breached. Shot blasting is the only practical method for complete mill scale removal.

When is wire brushing acceptable for steel surface preparation?

Wire brushing is acceptable for maintenance painting over intact coatings, treating inaccessible areas as a supplement to shot blasting, low-risk indoor environments with minimal corrosion exposure, and non-critical structures with a short planned service life. It is not acceptable where a coating specification requires Sa 2.5 or where coating manufacturer warranties are required.

How long does a coating last on a wire-brushed surface vs a shot-blasted surface?

A coating applied over a wire-brushed surface typically lasts 3–7 years before failure. The same coating applied over a shot-blasted surface typically lasts 15–25 years. The higher upfront cost of shot blasting is recovered many times over in extended coating life and reduced maintenance frequency.

What surface profile does shot blasting produce?

Shot blasting with steel grit or mixed media typically produces a surface profile of Rz 40–70 µm (40–70 microns peak-to-trough). This is the profile required by most industrial coating systems including epoxy, zinc-rich primer, and intumescent coatings. Wire brushing produces no measurable surface profile.

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Expert article on shot blasting and surface preparation from Commercial Shot Blasting — UK mobile shot blasting contractors.

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