Effective surface preparation is critical to ensure maximum coating performance. Each stage of the process can be accurately measured, from the climatic conditions prior to work commencing, through to the chloride content of the abrasive, as well as the blast profile, cleanliness, and wet and dry film thickness. However, certain applications, due to their nature, limit the possibilities of measuring the parameters associated with the successful surface preparation and coating application. One such application is the internal blasting of pipelines.
The two critical measurable parameters involved in surface preparation are surface cleanliness and surface profile. Numerous standards can be applied to surface cleanliness – however for this article will use the Swedish Association (SA) standards. By applying the SA standards, a contractor can identify to which degree of cleanliness he must prepare the surface. The standard most frequently specified by paint manufacturers is SA 2.5 – Near White Metal Blast Cleaning. This is defined as: “Removal of nearly all mill scale, rust, rust scale, paint, or foreign matter by the use of abrasive 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 light streaks or slight discolorations caused by rust stain, mill scale oxides, or light, tight residuals of paint or coatings that remain. At least 95% of each square inch of surface are shall be free from all visible residues, and the remainder shall be limited to the light discoloration mentioned above.
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