What Is Sandblasting? Sa 2.5 Surface Preparation and Coating Durability

The life of a coating system is usually decided not by the paint itself but by how well the surface beneath it was prepared. Even the best paint applied over rust, mill scale or old coating residue will soon blister and peel, and corrosion will start again at the same spot. Sandblasting is the mechanical surface cleaning method that removes this risk.

Cleanliness Grades

Surface preparation follows the internationally recognised ISO 8501-1 standard, which classifies cleanliness from Sa 1 to Sa 3: Sa 1 (light blast cleaning, removing loose rust and dirt), Sa 2 (thorough blast cleaning, except for firmly adhering rust residue), Sa 2.5 (very thorough blast cleaning; almost all visible rust, mill scale and foreign matter is removed — the grade most often required for heavy industrial plants, tanks and outdoor steel structures) and Sa 3 (blast cleaning to visually clean steel, the highest level).

Types of Abrasive

Although it is called “sandblasting”, natural sand is mostly no longer used — it is restricted in many countries because of the risk of silicosis from inhalation. Abrasives such as garnet, aluminium oxide or steel grit/shot are used instead, chosen according to the surface and the required roughness profile.

Dry or Dust-Free?

Conventional dry blasting is the most common and fastest method; for delicate or enclosed areas, dust-free or wet blasting techniques that largely eliminate dust are preferred.

Timing Is Critical

A freshly blasted steel surface quickly starts to rust again with the moisture in the air (flash rust). That is why the time between blasting and priming is kept short — the longer the cleaned surface stays exposed, the more of the achieved surface quality is lost.