Which cleaning standard signifies the least level of surface cleanliness?

The choice indicating that SA 1 signifies the least level of surface cleanliness is accurate because SA 1 is defined by the Swedish Standard SS-ISO 8501-1 as “lightly cleaned steel.” This standard allows for some rust, mill scale, and contaminants to remain on the surface, provided they do not significantly affect the adhesion of the subsequent coating. It requires only that loose surface contamination be removed, which may be done with methods like hand tools or power tools.

In contrast, the other standards—SA 2, SA 2 1/2, and SA 3—represent progressively cleaner levels. SA 2 denotes “thoroughly cleaned steel,” which requires a higher degree of cleanliness, needing more extensive removal of rust and contaminants. SA 2 1/2 involves “very thoroughly cleaned steel,” allowing for minimal visible contaminants, while SA 3 refers to “completely clean steel,” indicating that the surface should be free of all contaminants and have a spotless appearance.

Thus, SA 1 is positioned as the standard that signifies the least level of cleanliness, which is why this choice is the correct response.