DIY Guide: How to Fix Rust Underneath Your Car

Rust on your car’s undercarriage is a common problem, especially in areas with harsh winters or salty roads. Dealerships and mechanics often charge hefty prices for rust repair, leading many car owners to seek a more affordable solution. If you’re facing this issue, you might be wondering how to fix rust underneath your car yourself. This guide provides a step-by-step approach to tackle undercarriage rust at home, saving you money and extending the life of your vehicle.

Please note: While this guide aims to be helpful, DIY car repairs come with risks. If you are not comfortable working on your car or dealing with tools and chemicals, it’s always best to consult a professional. Safety precautions are crucial, and we are not responsible for any outcomes resulting from attempting this repair.

Step-by-Step Guide to Repairing Undercarriage Rust

Step 1: Initial Assessment and Preparation

Begin by peeling back the carpet in the area affected by rust. This usually involves removing plastic trims and clips that hold the carpet in place. Carefully detach these components to access the rusted metal underneath.

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Once the carpet is removed, thoroughly assess the extent of the rust damage. Determine which areas of the metal are salvageable and which parts need to be removed. The goal is to grind away the severely rusted sections, leaving you with a clean, workable area. Aim for a rectangular or square shape when cutting out the rust, as these shapes are easier to patch and repair.

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Step 2: Rust Removal and Surface Treatment

After assessing the damage, proceed to remove the loose and excessive rust. Use a grinder or wire brush to clean the affected area down to bare metal. Once the loose rust is removed, apply a rust reformer product, such as Rustoleum Rust Reformer, to the entire rusted area. This treatment will neutralize any remaining rust and prevent further corrosion. Apply the rust reformer both inside the car and underneath it for comprehensive protection.

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Ensure you follow the manufacturer’s instructions for drying time before proceeding to the next step. Proper rust treatment is crucial for a lasting repair and to prevent the rust from returning.

Step 3: Patching the Rust Hole

For patching the hole, you will need thin metal sheets. Galvanized steel or aluminum sheets from a home improvement store like Home Depot are suitable. Measure the dimensions of the hole you created and cut out two metal pieces – one for the inside patch and one for the underside patch. Make sure each piece is approximately half an inch wider and longer than the hole to ensure sufficient overlap for bonding and sealing.

Master Flow 24 in. x 36 in. Galvanized Steel Flat Sheet – GFS24X361P at The Home Depot

Step 4: Bonding and Securing the Patches

Now it’s time to apply body filler, such as Bondo, to adhere the metal patches. For the underside patch, apply Bondo-Hair filler with long strand reinforced fiberglass to the metal sheet, covering it entirely except for a half-inch border around the edges. Press this patch firmly onto the underside of the car over the rusted area. You will need to hold it in place or use supports until the Bondo cures and hardens, as it can generate heat during this process.

3M/1 qt. Bondo-Hair filler with long strand reinforced fiberglass (762) | Body Filler | AutoZone.com

For the inside patch, follow a similar procedure, but apply a generous amount of Bondo in the center of the metal sheet. This ensures that there are no gaps between the two metal sheets once they are in place. Again, secure the patch until the Bondo is fully cured.

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Step 5: Welding or Additional Bonding and Protection

To further secure the metal patches, welding is highly recommended. Weld the edges of both the inside and underside metal sheets to the car’s frame. Important safety note: Do not weld galvanized metal as it produces toxic zinc fumes. If welding galvanized steel, ensure proper ventilation and respiratory protection. Alternatively, use steel sheets for welding. Always wear appropriate safety gear, including welding glasses and gloves, when welding.

If welding is not an option, you can use additional Bondo, this time short strand variety, around the edges of the metal patches to ensure a strong bond and seal. After the patches are secured and cooled, apply a rubberized undercoating spray to both the inside and underside repairs, and to the surrounding areas. This undercoating will protect the new metal from moisture, road debris, and further corrosion. Apply at least two coats, allowing each coat to dry for a few minutes in between.

3M? Professional Grade Rubberized Undercoating, 03584, 16 oz., 6 per case

Step 6: Final Touches

Let the rubberized undercoating air dry completely for at least an hour. Once dry, you can reinstall the carpet and plastic trims, returning the interior of your car to its original state.

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Important Notes for DIY Rust Repair

• Rust Severity: If the rust is only on the undercarriage surface and hasn’t penetrated through the interior, you can skip the steps involving interior repair.
• Bondo Odor: Be aware that Bondo has a strong odor that can linger for some time after application. Ventilation can help reduce the smell over time.
• Working Time with Bondo: Bondo hardens very quickly once mixed with the hardener. Work in small batches and apply it within 3 minutes of mixing to avoid waste.
• Safety Gear: Always wear safety goggles when grinding or sanding rust to protect your eyes. Gloves are highly recommended throughout the entire process to protect your skin from chemicals and debris.
• Undercoating Layers: Applying multiple coats of rubberized undercoating not only provides better protection but also helps reduce road noise.
• Welding Safety: Reiterate the warning against welding galvanized metal due to toxic fumes. Ensure proper safety measures if you choose to weld.

By following these steps, you can effectively fix rust underneath your car and prevent further damage. This DIY approach can save you significant money compared to professional repairs and help maintain your vehicle’s structural integrity for longer.