Proper surface preparation before paint application is vital to ensure long-term protection given by the painting system. Therefore, prior to commencing work the initial conditions of the steel surface need to be assessed.
Steel surfaces that have never been protected by paint coating may be rusty, with mill scale or other contaminants (dust, lubricants, ionic contaminantion/soluble salts, sediments etc.)
Abrasive blasting is the best mechanical method of surface preparation, which uses a wide range of abrasives. According to the PN-ISO 8501-1 norm in case of abrasive blasting of steel constructions the following grades are distinguished
Sa 1 – Light blasting-cleaning
When inspected with the naked eye, the surface cannot possess any visible oil, lubricants, scale that does not adhere well, rust, paint coating or other contaminants.
Sa 2 – Thorough blast-cleaning
When inspected with the naked eye, the surface cannot possess any oil oil, lubricants, dust, bigger scale, rust, paint coating or other contaminants. All residue contaminants adhere to the surface.
Sa 2 ½ – Very thorough blast- cleaning
When inspected with the naked eye, the surface cannot have any oil, lubricants, dust, scale, rust, paint coating or other contaminants. The only residue contaminants are in the form of spots, drops or stripes.
Sa 3 – Blast-cleaning to visually clean steel
When inspected with the naked eye, the surface cannot be covered with oil, lubricants, dust, scale, rust, paint coating or other contaminants. The surface must have the same metallic shade.
Preparing the zinc surface before painting determines the quality and durability of the DUPLEX protection. Hot-dip galvanized surfaces are best cleaned with light blasting in order to remove oxide layers, any contaminants and to improve surface adhesion through better roughness. Unremoved contaminants may greatly worsen paint adhesion to the surface, especially if they are contaminants of cooling baths on new zinc coating.
Everthing that disturbs proper zinc surface moisture with paint and, because of this, the adhesion of ready coating to the surface reduces the expected protective properties. Applying liquid paints on zinc coating without surface preparation is a great risk and frequently leads to peeling-off. In such a situation, apart from lower protective qualities, there is also the problem of unaesthetic surface.