Steel trowel/steel float, which is especially good for spreading wall and floor compounds.

Steel trowels/finishing floats are meant for spreading compounds evenly over large areas. They're available in several different sizes. With bigger floats, you can spread larger amounts of compound faster, but the bigger the float, the heavier the spreading work gets too.

Compounds are spread on the surface by pulling or pushing the float against it. This way, the uneven spots on the surface get filled evenly.