Pre-treatment in depth: blasting and degreasing
The invisible work that determines whether the coating lasts for years
When a coating peels off a couple of years too early it's rarely the paint itself that's wrong. As a surface treatment worker you quickly learn that the work nobody sees afterward — the pre-treatment — is what determines whether the coating holds for twenty years or falls off within two. An often-cited rule of thumb in the coating industry notably highlighted by manufacturers like Sherwin-Williams says that up to 80 percent of early coating failures can be traced back to insufficient pre-treatment.
§Cleanliness grades in ISO 8501-1
The standard ISO 8501-1 establishes how to visually assess the cleanliness of a steel surface before painting – both rust grades on untreated steel and the cleanliness grades a surface can achieve through blast cleaning. The most well-known part of the standard is the Sa scale. The prefix 'Sa' comes from the word 'sand', because the reference panels for the standard were originally produced by sandblasting.
| Degree | Name | What is required? |
|---|---|---|
| Sa1 | Easy air purging | Free from loose rust, loose paint, dirt and grease |
| Sa2 | Thorough blower cleaning | Free from virtually all loose material; most of the mill scale and rust is gone |
| Sa2½ | Very thorough blower cleaning | Uniform grayish-white surface only very faint shadows from rust or old coating remaining |
| Sa3 | Blast cleaning to visibly clean steel | Visually completely clean uniform metallic surface without visible residues |
§The surface profile is just as important as the cleanliness
Purity is only half the story. Blast cleaning also creates a surface profile – a microscopic roughness that gives the coating something to grip mechanically. If the profile is too smooth, the coating sticks poorly. If it's too coarse relative to the planned layer thickness, the highest peaks in the profile risk being only thinly covered, which gives early rust spots even though the rest of the surface looks fine.
§Degreasing must be done before the workpiece is blast cleaned
Fat and oil must be removed before mechanical processing starts — otherwise blasting or grinding merely spreads the fat film across the entire surface instead of removing it. Defatting agents are typically alkaline or solvent-based, precisely because they can effectively break down and dissolve fat and oil films, which water alone cannot handle.
§Phosphating — a chemical intermediate layer
On many industrial and powder coating tasks, mechanical surface preparation is followed by phosphating: a chemical conversion treatment, often with zinc phosphate, that converts part of the metal surface into a phosphate layer. This improves the adhesion of the subsequent coating and puts an extra, corrosion-inhibiting layer between steel and paint.
§The consequence of skipping a step
The temptation to save time on surface preparation is real, because it is the part of the job the customer never sees afterwards. But studies from the coating industry suggest that fixing a failed coating can cost many times more than correct surface preparation from the start. The experienced surface treater knows that time spent on cleanliness, surface profile, degreasing and any phosphating is the best investment in the entire task.