pre-treatment process for the deep cleaning of artifacts.
Sandblasting is an alternative or parallel process to chemical pre-treatment, it is necessary before painting if there are oxidations, encrustations, traces of welding, old paint layers and unwanted imperfections on the artifacts. By sandblasting, a clean and homogeneous surface is obtained, maintaining a suitable roughness to favor the adhesion of the paint. The roughness level is determined by the degree of sandblasting performed; it must always be balanced: a too low roughness leads a less adhesion of the paint, a too high roughness involves the use of a greater amount of paint to uniformly cover the entire surface.
Sandblasting can be carried out on both ferrous materials and non-ferrous alloys (for example aluminum) and galvanizing, and is suitable for both small parts (inside protective chambers) and for large structures, frames and carpentry. The final result on the product is variable according to the customer's requests, acting between execution times and pressure of the shot.
To know which degree of sandblasting to undertake, an inspection must be carried out based on standard evaluation criteria, described in EN ISO 8501 "Preparation of steel substrates before the application of paints and related products" which lists the degrees of rust (from A to D) and the methods to be applied before sandblasting, such as scraping and brushing.
The degrees of sandblasting are the follows:
Sandblasting Sa 1 is a coarse and light sandblasting, it consists in the removal of rust flakes and visibly detached impurities. Only the most tenacious rust remains (which still guarantees good adhesion of the paints). The surface must be free of oil, grease and visible dirt.
Sandblasting Sa 2 is a commercial sandblasting, quite marked and heavy, it consists of a thorough cleaning and therefore the surface is free of foreign residues. This degree of sandblasting requires the removal of all rust, calamine and other foreign matter on the surface of the metal. The surface obtained is not particularly uniform, it assumes a mostly gray color and a roughness that lends itself well to a firm adhesion of the paint layers that will be applied.
Sandblasting Sa 2.5 is an almost white metal sandblasting, with a very heavy jet, it consists in the removal of any impurities, corrosion products, dirt, calamine, rust, coatings and foreign materials present on the surface. Any residual traces of previous contamination should appear only as halos or light spots in the form of dots or streaks. Sandblasting Sa 2.5 compared to Sa 2 is more homogeneous, it is the most requested and adopted degree of finish.
Sandblasting Sa 3 is a very thorough, white metal sandblasting. It removes any impurities, residues, corrosion products and all foreign elements. The surface must have a uniform gray-white appearance, with an optimal roughness that allows perfect fixing of the paint to the metal.
The roughness resulting from the sandblasting operations must be checked with special tools such as roughness comparators and roughness meters.
Sand was once the most commonly used abrasive material for this treatment. Nowadays other materials are also used, such as steel grit, copper slag, powdered abrasives, walnut shells, coconut shells.
Appropriate safety devices must be used during the blasting process. Abrasive particles can irritate the eyes and skin and can cause silicosis if inhaled.
The sandblasting operation takes place after
the scraping and brushing operation where coarse dirt, grease, impurities and
heavier layers of rust are removed which create instability and poor adhesion
of the film on metal.