Surface preparation standards

The performance of a protective coating depends largely on its ability to adhere to the substrate. Contaminants —mill scale, rust, grease, salts— come between the paint and the steel and ruin that adhesion, encouraging corrosion under the film.

That is why the standard sets a target cleanliness grade. Hitting exactly the specified grade matters: falling short leads to premature coating failure, and exceeding it raises the cost with no added benefit.

Surface preparation work is standardised by several international associations. The most widely used in Latin America are the American SSPC (Steel Structures Painting Council) and the European standard, originally the Swedish SIS 05 5900. Most rely on visually comparing the treated surface against photographic references.

A note on current status: in 2021 SSPC and NACE merged into AMPP (Association for Materials Protection and Performance), which maintains the combined library; the standard numbers were kept, so specifications still read “SSPC-SP 10” or “NACE No. 2” unchanged. In parallel, the former SIS 05 5900 evolved into the international standard ISO 8501, today the reference for visual grades of rust and cleanliness.

The starting point also matters. The standards define four rust grades of the steel before treatment:

• Grade A: surface with the mill scale intact and practically no corrosion.
• Grade B: corrosion has begun and the mill scale is starting to detach.
• Grade C: the mill scale has been removed by corrosion or flakes off when scraped; no major visible pitting yet.
• Grade D: the mill scale is gone through corrosion and there is large-scale visible pitting.

The job is specified by combining the starting point and the finish. For example, starting from rust grade B and reaching Sa 2½ is designated “B Sa 2½”.

Beyond blasting, the standards cover other preparation methods, often combined:

• Solvent cleaning (SP 1): removes oil and grease; the step before almost all the others.
• Hand and power tools (SP 2, SP 3, SP 11, SP 15): brushes, abrasives and electric or pneumatic tools, with bare-metal or commercial grades and a minimum 25 µm profile.
• Chemical pickling (SP 8): by chemical reaction; acceptable results but a higher-risk method.
• Pressure water cleaning (waterjetting): the former SP 12 was rewritten and replaced by standards WJ 1 to WJ 4.
• Concrete (SP 13) and non-ferrous metals (SP 16): with a minimum 19 µm profile on non-ferrous metals.

A few field keys that prevent costly mistakes:
• The most used grade is Sa 2½ / SP 10: it provides enough anchoring for most uses, without the cost of white metal (Sa 3 / SP 5).
• Visual comparison is not exact: the appearance and colour of the blasted surface vary with the abrasive (sand, steel shot) and the base metal. When in doubt, refer to the degree of rust removal the standard describes, not just the photo.
• It is worth preparing test panels of the same material, with the actual abrasive and field conditions; once approved, they serve as the comparison reference throughout the job.
• Hitting the exact grade is what counts: below it, the coating fails early; above it, you pay more without gaining performance.

Surface preparation standards translate a goal —that the coating adheres and lasts— into a measurable, verifiable cleanliness grade. Knowing the equivalences between SSPC, ISO and NACE, the initial rust grades and the blast cleaning grades makes it possible to specify precisely and control the work on site. In practice, Sa 2½ / SP 10 covers most projects; the rest is choosing the right abrasive and verifying the result.