There should be a set of samples that illustrate the different levels. You just match the surface with the samples.

This is the standard we use in the U.S.

"SSPC VIS1-89" Visual Standard for Abrasive Blast Cleaned Steel, (Standard

Reference Photographs), Guide and Standard, SSPC: The Society for Protective

Coatings, (formerly Steel Structures Painting Council) 40 24th Street, 6th Floor,

Pittsburgh, PA 15222-4656, (412) 281-2331; fax (412) 281-9982.

SSPC-SP-10
Sa 2-1/2
NACE 2

Near-White Blast Cleaning - Removal of nearly all mill scale, rust, rust scale, paint, or foreign matter by the use of abrasives propelled through nozzles or by centrifugal wheels, to the degree hereafter specified. A Near-White Blast Cleaned Surface Finish is defined as one from which all oil, grease, dirt, mill scale, rust, corrosion products, oxides, paint or other foreign matter have been completely removed from the surface except for very light shadows, very slight streaks or slight discolorations caused by rust stain, mill scale oxides, or light, tight residues of paint or coating that may remain. At least 95 percent of each square inch of surface area shall be free of all visible residues, and the remainder shall be limited to the light discoloration mentioned above.

ISO 8501-1 Sa 2.5 does not quote any percentage for visible residues and remainder.

An SA 2.5 is a near-white surface which means the galvanizing will also be removed in the sand blasting process.