Wear gloves, eye protection, and a respirator
Clean the repair area thoroughly
Remove rust, paint, grease, and loose material
Sand the surface to create a rough profile
Mix the body filler with the hardener on a clean mixing board
Blend until the color is uniform
Apply the filler in thin layers with a spreader
Shape it before it hardens
Let it cure fully
Sand the hardened filler with progressively finer grits
Reapply filler if needed to fill low spots
Finish with primer before painting