Nov 09 2022
A properly sandblasted surface is essential prior to any coating system that will be used on metals. The fabricated metal cannot be finished until it is properly cleaned through sandblasting. The preparation will remove any contamination which affects adhesion or could cause corrosion. Oil, grease, rust, and scale, as well as soluble salts must all be removed. Contaminants such as oil, grease, and other chemicals must be removed before sandblasting, usually with solvent soaked rags. After the initial cleaning, sandblasting will take care of the surface contaminants remaining so all future finishes will adhere to the metal and the final coating will hold through any conditions.
Standards for Sandblasting
The Society for Protective Coatings (SSPC) and NACE International have issued joint standards for blast cleaning (sandblasting) to achieve varied degrees of steel surface cleaning before a finishing coat is applied. The level of cleaning depends on both the metal requiring finishing and the finishing coat itself. Sandblasting is done using blast nozzles with compressed air and an abrasive. The evaluation to meet the standards should always be done objectively to ensure compliance with the requirements.
Standards and Requirements for SP6 Sandblasting
SP6 sandblasting is referred to as Commercial Blast and is the most widely specified abrasive blast preparation. When a clean surface is required, the SP6 method of sandblasting requires removing all rust, mill scale, dirt, and other contaminants on the surface. The sandblasted surface will be a gray color, but some “shadows” from a tightly-adherent previous coating are tolerated, as long as at least two-thirds of the surface area is free of all visible residue. Rather than an overall two-thirds, each square inch must have two-thirds of the surface area clean. For most metal environments, SP6 requirements will meet the standards for finishing.
Standards and Requirements for SP10 Sandblasting
SP10 sandblasting, Near White, is a step beyond SP6 and only very light shadows from rust, mill scale, oxides, or previous paint are permitted. The specifications for SP10 require a minimum of 95% of the surface to be free of all visible discoloration. This is the best quality surface preparation that can be expected for plant facility maintenance work. SP10 is primarily used in areas exposed to high humidity, chemical atmospheres, marine, and other corrosive environments. The harsh conditions require a high degree of adherence for the finishing materials, and this adherence can be achieved only through thorough sandblasting. In some cases, a surface dust test will be required to make sure all dust has been removed from the surface, and a replica film test will measure the surface profile to ensure that 95% of each square inch has been blasted clean.
When comparing SP6 and SP10 sandblasting, the most notable difference is the degree of cleanliness required from the sandblasting and the environment in which the fabricated metal will be installed. The coating manufacturer will specify the surface profile required for adhesion for the particular metal and finish. The overall goal is to never attempt to finish a dirty surface. The standards of preparation are there to ensure a quality finish. Not all fabrication shops have the in-house capacity for finishing. We are proud to offer exceptional welding and fabrication services, and we have a finishing department in house. For a quote on your next fabrication, contact Swanton Welding.