Surface preparation for structural steel

Appropriate surface preparation is a vital part of the painting process in steel structure. Steel surfaces need to be treated to prevent corrosion over the course of an asset’s lifetime. Surface preparation is the first step in this process.

For primers and paints to work as intended, structural steel must go through a surface preparation process. Specifying the correct level of surface preparation is key to achieving the finished look you want. When contaminants such as dirt, rust, salts, or oil are left on steel surface and painted over, they can draw in moisture, which will chip the paint and rust the steel. 

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The most common cause of painting and coating failure is improper surface preparation. A level of surface preparation that complements the paint will uniformly adhere to the steel and last for the intended service life.

Due to varying attributes and performance characteristics of the many coating types, surface preparation requirements can also be unique from project to project. SSPC and NACE have set industry standards for surface preparation that are detailed below.

SP1	Solvent Cleaning	Removal of all visible oil, grease, dirt, soil, salts, and contaminants by cleaning with solvent, vapor, alkali, emulsion, or steam.
SP2	Hand Tool Cleaning	Removal of loose rust, loose mill scale, and loose paint, by hand chipping, scraping, sanding, and wire brushing.
SP3	Power Tool Cleaning	Removal of all loose rust, loose mill scale, and loose paint, by power tool chipping, descaling, sanding, wire brushing, and grinding.
SP 5/NACE No. 1	White Metal Blast Cleaning	Removal of all visible rust, mill scale, paint, and foreign matter by blast cleaning by wheel or nozzle (dry or wet) using sand, grit or shot. For very corrosive atmospheres where high cost of cleaning is warranted.
SP 6/ NACE No. 3	Commercial Blast Cleaning	Removal of all visible rust, mill scale, paint, and foreign matter by blast cleaning. For conditions where a thoroughly cleaned surface is required.
SP 7/ NACE No. 4	Brush-Off Blast Cleaning	Removal of all loose contaminants while uniformly roughening the surface. Tightly adhering residues of mill scale, rust, and coatings may remain.
SP 8	Pickling	Complete removal of rust and mill scale by acid pickling, duplex pickling, or electrolytic pickling.
SP 10/NACE No. 2	Near-White Blast Cleaning	Removal of all visible rust, mill scale, paint, and foreign matter by blast cleaning. For high humidity, chemical atmosphere, marine, or other corrosive environments.
SP 11	Power Tool Cleaning to Bare Metal	Complete removal of all visible oil, grease, coatings, rust, corrosion products mill scale, and other foreign matter by power tools, with resultant minimum surface profile.

Surface preparation is an important step for a range of materials ahead of coating, welding, and other processes. There are a range of different techniques that range in effectiveness and cost. Generally speaking, the more complete the cleaning, the more expensive the technique.

In some cases, several surface preparation techniques need to be combined, such as through the use of solvent cleaning to remove oils ahead of abrasive cleaning, followed by jet washing to remove left-over dust particles.