Wash and dry the area thoroughly
Remove loose rust, paint, and debris with a wire brush, sanding disc, or grinder
Sand the rusted area down to clean metal
Treat any remaining rust with a rust converter or rust remover
Clean the surface with wax and grease remover
Apply body filler if needed to smooth pitted areas
Sand the filler smooth
Apply a self-etching or rust-inhibiting primer
Paint the repaired area with matching automotive paint
Apply clear coat if required
Seal seams and edges with automotive seam sealer if needed
Undercoat exposed underside areas for protection
Replace panels if rust has caused structural damage or holes too large to repair
Inspect and repair hidden rust sources such as drains, weatherstripping, and leaks
Keep the car clean and dry to prevent rust from returning