Since rust is a loose, porous and constantly developing and expanding material, if it is not removed and painted on top of it, the development and expansion of rust will deform the painted surface and produce tiny ruptures to accelerate the entry of water, oxygen and corrosive ions, thus greatly affecting the anti-corrosion effect of the paint film and causing the life of steel to be greatly reduced.
Rust removal before painting is the key process of steel anti-corrosion coating. At present, the effective rust removal method in practice is the physical method to remove the rust from the steel surface. Rust removal grades are divided into three types: spray or blast rust removal (Sa), manual and electric rust removal (St), and flame rust removal (FI).
The steel surface treatment of spray and blast descaling, indicated by the letter “Sa”, is divided into four grades.
Sa1 – light jet and shot descaling. The steel surface is free of visible grease, dirt, unattached oxide, rust, paint coating and other adhesions.
Sa2 – Thorough jet or blast descaling. No visible grease, dirt, oxidized skin, rust and other adhesions on the steel surface are basically removed.
Sa2.5 – Very thorough jet or blast descaling. There is no visible grease, dirt, oxide, rust, paint coating and other adhesions on the steel surface, and any residual trace is only a slight discoloration in the form of dots or streaks.
Sa3 – Jet or blast descaling that makes the steel surface very clean. The steel surface is free of visible grease, dirt, oxide, rust, paint coating and other adhesions, and this surface shows uniform metallic color.
Steel surface treatment manual rust removal grade.
St2 – Thorough hand and power tool descaling The steel surface should be free of visible grease and dirt and free of adherents such as poorly attached oxidation, rust and paint coatings.
St3 – Very thorough hand and power tool descaling should be free of visible grease and dirt and free of adherents such as poorly adhered oxidation, rust and paint layers. Rust removal should be more thorough than St2, and the surface of the exposed part of the substrate should have a metallic luster.
Steel surface treatment flame descaling.
FI – Flame rust removal steel surface is free of adhesions such as oxidation, rust and paint coating. Any residual traces shall be the original color of the surface (dark shadows of different colors)