Which tool is commonly used to apply filler in auto body repairs?

A plastic spreader or trowel is commonly used to apply filler in auto body repairs because of its design that allows for smooth application and even spreading of the filler material. The flexible blade contours to the surface being worked on, allowing for better control and a smoother finish. This tool is especially effective when working with materials like body filler, as it minimizes the chances of introducing air bubbles and ensures that the filler is evenly applied across the damaged area.

In contrast, a bristle brush is typically not used for applying filler since brushes are more suited for painting or coating applications rather than spreading thick materials like body filler. A power sander is used for smoothing and shaping cured filler rather than applying it, and a putty knife, while it can be used for similar purposes, lacks the flexibility and optimal design that a plastic spreader offers for this specific application. Thus, the plastic spreader stands out as the most effective tool for applying filler in auto body repairs.