1. Identify the Extent of Rust Damage

• Thoroughly inspect the entire chassis, focusing on common rust-prone areas such as the rocker panels, floor pans, wheel wells, subframes, and suspension mounts.
• Use a flashlight and a mirror to check hidden areas and behind plastic trims.
• If necessary, use a rust converter or scraper to remove surface rust and reveal the extent of the damage underneath.

2. Prepare the Work Area and Gather Tools

• Choose a well-ventilated area to work on, preferably with moderate temperatures and low humidity.
• Gather necessary tools and materials including sandpaper, wire brushes, rust converters, primers, welding equipment (if needed), metal patches, and paint.

3. Remove the Rust

• For minor surface rust, use sandpaper or a wire brush to scrub away the rust until you reach bare metal.
• For more severe rust, consider using a grinder to remove extensive rust, but take care not to damage healthy metal.
• If the rust has perforated the metal, cut out the affected area using a cutting tool such as an angle grinder.

4. Treat the Cleaned Areas

• After removing rust, clean the area with a degreaser to ensure no grease or debris is left.
• Apply a rust converter to the cleaned area to stop any remaining rust from spreading.

5. Apply a Metal Patch or Fiberglass

• For small holes, a reinforced fiberglass patch can be applied using resin.
• For larger holes or structural components, cut a piece of sheet metal to size and weld it in place. Ensure the new metal is of the same gauge as the original chassis metal.
• Grind down any welds or edges to create a smooth surface.

6. Prime and Paint the Repaired Area

• Apply a rust-inhibiting primer to the bare metal or patched area to prevent future rusting.
• Once the primer is dry, apply a coat of paint that matches your vehicle's color. For added protection, apply an undercoating or sealant to the underside of the car.

7. Prevent Future Rust

• Regularly wash your car, especially the undercarriage, to remove dirt and salt that can cause rust.
• Apply a rust-proofing treatment or coating to the undercarriage periodically.
• Inspect your car regularly for signs of rust and address any issues early before they become severe.