Sandblasting

Sandblasting is the most widely used surface preparation for steel surfaces. Sandblasting removes rust, existing coatings, mill scale, dirt, oil, grease, and other contaminants. It creates an anchor pattern for the primer.

Sandblasting does not remove all the salt from the substrate surface. Before sandblasting newly manufactured or repainted steel, pressure washing or steam cleaning is necessary. Salt Test Kits are available for determining the cleanliness of the surface.

There are standards for the degree of cleanliness of the sandblasted steel substrate. All paint specifications that include sandblasting should specify the type of blast required. The standards currently in use follow the guidelines provided by NACE (National Association of Corrosion Engineers), SSPC (Steel Structures Painting Council) or Sa (Swedish Pictorial Standards).

Commercial Blast:

NACE 3, SSPC-SP6, or Sa 2, Is a method of preparing steel surfaces to be free of all visible oil, grease, dirt, dust, mill scale, rust and paint. Generally, evenly dispersed, very light shadows, streaks and discolouration caused by stains of rust, stains of mill scale, and stains of previously applied paint may remain on no more than 33% of the surface. Slight residues of rust and paint may also be left in the craters of pits if the original surface is pitted.

Near White Metal Blast:

NACE 2, SSPC-SP10, or Sa 2 1/2, Is a method of preparing steel surfaces to be free of all visible oil, grease, dirt, dust, mill scale, rust and paint. Generally, evenly dispersed, very light shadows, streaks and discolouration caused by stains of rust, stains of mill scale, and stains of previously applied paint may remain on no more than 5% of the surface.

White Metal Blast:

NACE 1, SSPC-SP5, or Sa 3, It is a method of preparing steel surfaces which, when viewed without magnification shall be free of all visible oil, grease, dirt, dust, mill scale, rust and paint/coatings. Sand or grit used in sandblasting is graded by mesh size. In Alberta, sand known as Sil 7 Sand (mesh size 16 – 30) is commonly used because of its aggressive and productive removal of paint, coatings, mill scale, rust, etc. This particular mesh size sand can create a profile on steel as high as 3.5 – 4.0 mils.

The sandblast profile:

The height from the top of the peaks to the depth of the pits, is a consideration when choosing a primer. A common practice is to recommend a primer that can easily be applied to a dry film thickness that exceeds the height of the profile.