Which grit level is least effective for smoothing the nail surface?

The grit level of an abrasive file plays a crucial role in nail care and finishing. A grit of 100 or lower is coarser and is typically used for shaping or shortening the nail rather than for smoothing the nail surface. This coarser grit can remove significant material quickly but does not provide the fine finish that is necessary for smoothing.

On the other hand, grits higher than 180 are designed for refining and polishing, which makes them more effective in achieving a smooth nail surface. The 150-180 grit range is suitable for some smoothing tasks but still might not provide the same level of refinement as higher grits. Therefore, 100 grit or lower is indeed the least effective when it comes to smoothing the nail surface, as it is more suited for initial filing instead of creating a polished appearance.