How to Fix a Rusted Subframe

by Brandon Getty Images

Cars spend most of their time outside amidst the elements, so rust can quickly begin to form on nearly any part of the body. As the name implies, the subframe is a smaller metal support structure within a larger vehicle body. The subframe usually braces integral parts of the car, such as drivetrain, suspension or engine. Once it has been removed from the vehicle, holes caused by metal corrosion can be treated. There are a number of fiberglass tape and filler kits available to help you patch subframes that have rusted through.

Step 1

Don a pair of thick gloves to protect your hands from metal shards, rust residue and caked grease as you work.

Step 2

Trim away all rusted metal surrounding the hole using tin snips. Brush away all flakes of rust and other debris from the area surrounding the hole.

Step 3

Sand the area free of rust using a pad of 24-grit sandpaper. Work until you have sanded about 3 inches surrounding the circumference of the hole. Follow with a thorough sanding using a pad of 80-grit sandpaper. When you finish, rust-free metal should show through around the hole.

Step 4

Tap the inner circumference of the hole inward using gentle strikes with a ball peen hammer. This ensures that the patch sets against the metal firmly and smoothly.

Step 5

Take a piece of release film from the kit and cut it roughly 2 to 3 inches larger than the sanded area surrounding the hole. Hold the film over the hole and indicate with a pen where the sanded area begins.

Step 6

Cut a piece of fiberglass matte material so it is roughly 1-inch smaller than the sanded area. Use the mark you made on the release film for reference.

Step 7

Open the containers of filler and hardener and mix according to the printed instructions. Spread the mixture evenly onto the piece of release film. Place the smaller piece of fiberglass matte in the middle of the film, then spread a layer of the mixture over it.

Step 8

Apply the patch over the hole with the release film facing outward. Ensure that the fiberglass matte completely covers the hole upon application. Smooth the patch flat over the hole using the palm of your hand.

Step 9

Allow the patch to cure for the time suggested on the container. Then, peel away the release film to reveal the patched hole beneath.

Sand the patched area smooth using a pad of 80-grit sandpaper. The area can be painted if desired.


  • Ideal working temperature for most repair kits is between 60 degrees and 90 degrees Fahrenheit.
  • Subframes can be sanded and painted to protect them from rust in the future. If you are not experienced with automotive painting, contact a local body shop for an estimate.
  • Clean the underside of your vehicle regularly by placing a garden sprinkler beneath it. The force of the water eliminates grime and road salt, which contribute to the formation of rust.


  • Do not attempt to remove the subframe from your vehicle if you are inexperienced with automotive mechanics. Have it removed and re-installed by a competent mechanic.

Items you will need

  • Thick gloves
  • Tin snips
  • 24-grit sandpaper
  • 80-grit sandpaper
  • Ball peen hammer
  • Rust hole repair kit
  • Pen
  • Scissors

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