Sometimes, especially during the repair of hydraulic cylinders, running traces are detected on the piston rod or in the cylinder tube and the question arises: "Can I mount it again in this state and continue to use it, or should I replace the component, so that I don’t have to worry about consequential damages?" Here, the "fingernail test" can be a simple and generally meaningful solution. Slide your finger over the relevant area or over the scratch that is to be examined. If you don’t feel anything, the component is usually not badly damaged, which means it can still be used.