What is the function of a darby or magnesium float in finishing?

• A. To apply curing compound
• B. To smooth and level the surface after initial screed and bull float
• C. To texture the surface for slip resistance
• D. To mix additional aggregate into the surface

Answer: (B)

The main idea here is smoothing and leveling the surface after the slab has been screed and bull floated. A darby or magnesium float is used to strike off high spots and fill small voids, creating a flatter, more uniform plane. This step further refines the surface while it’s still plastic, making it ready for the final finish (whether that’s troweling for a smooth look or a light texture). Using this float too early or too aggressively can overwork the surface, but done correctly it produces a consistent, dense top layer.

The other tasks don’t fit this function: applying curing compound is done after finishing the surface is set, texturing for slip resistance is typically done later (or with a broom) to create a texture, and adding more aggregate would disrupt the finish rather than smooth and level it.