When can a scaffold plank be deemed unusable?

A scaffold plank is deemed unusable primarily when it is compromised in structural integrity or does not meet safe usage criteria. The option indicating that a plank was previously used as a sill suggests that it might not have the appropriate specifications or strength for use as a scaffold plank. Sills are typically designed to support different loads and applications; therefore, repurposing them without proper assessment could lead to safety concerns.

The other options do not indicate significant issues that would render a plank unusable. Painting a plank may affect visibility or maintenance, but it does not inherently compromise structural integrity. Minor damage might be repairable and not significant enough to deem the plank unusable, provided it is within acceptable limits according to safety standards. Lastly, the fact that a plank has been used once does not inherently affect its usability for future applications, as long as it remains in good condition after that initial use. Thus, identifying a plank as unusable hinges on its structural suitability for the intended scaffolding purpose, which is appropriately addressed by the scenario of it being used as a sill.