In SSPC-SP6/NACE 3, staining is limited to what percentage per each unit area?

In SSPC-SP6/NACE 3, the standard outlines the requirements for commercial blast cleaning of steel surfaces prior to coating application. The guideline regarding staining indicates that for surfaces to be considered adequately prepared, any staining must be limited to a maximum of 33% of each unit area. This level of allowance is put in place to ensure that the steel surface has been sufficiently cleaned of contaminants like rust, mill scale, and other debris, which can adversely affect coating adhesion and performance.

A maximum staining allowance of 33% strikes a balance between ensuring that most of the surface is clean, while recognizing that complete elimination of all contaminants can be challenging in practical settings. This ensures that the surface meets the necessary clean surface requirements for proper coating adhesion, without being overly stringent.

In evaluating the other options, it is imperative to recognize that the lower percentages (like 10%, 20%, and 5%) would not conform to the SSPC-SP6/NACE 3 standards. Therefore, the 33% figure is aligned with the guidelines aimed at achieving effective surface preparation for protective coatings while allowing for some reasonable level of imperfection during the cleaning process.