All metal structures are subject to corrosion phenomena, which become intense depending on the environmental conditions and the use they are subject to. Proper preparation of the surfaces prior to carrying out the painting cycles is essential to ensure effectiveness and durability.
With this in mind, the best method is sandblasting with appropriate abrasive materials (silica sand, garnet or corundum) in order to remove oxides and salts, rust, scale, flaky surfaces and mill scales or traces of old paint that may compromise the adhesion of new paints.
The degree of blasting, according to the commonly used Swedish standards, is as follows:
- White metal sandblasting (Sa3) which consists of the removal of any impurities and corrosion products thus obtaining uniform gray-white surfaces;
- Almost white metal sandblasting (Sa2½) which consists of the removal of most impurities and corrosion products thus obtaining gray surfaces with a non consistent appearance;
- Commercial sandblasting (Sa2) similar to almost white metal but more irregular;
- Coarse sandblasting (Sa1) which consists of the removal of detached impurities while leaving the impurities that adhere well and ensure good paint adhesion.
The choice of the grading depends on both the state of the structures to be treated and the painting cycle to be carried out.
Main standard references
UNI EN ISO 8501-1:2012: preparation of steel substrates prior to applying the paint and related products – visual evaluation of the surface cleaning degree – degree of rusting and preparation of uncoated steel substrates or steel substrates after total removal of the pre-existing coating.
UNI EN ISO 8501-1:2012: degree of preparation of steel surfaces already coated after local removal of the previous coatings.
UNI EN ISO 8501-3:2008: degrees of preparation of welding, edges and other areas with superficial imperfections.
UNI EN ISO 8501-4:2012: : surface conditions, degree of preparation and degree of rust following high pressure water spraying.
UNI EN ISO 11124:1999: preparation of steel substrates prior to applying paints and similar products – abrasive metal requirements for sandblasting.