How to Find and Fix Hidden Body Rust

Rust is often a silent, invisible enemy lurking in places that are easy to overlook during routine maintenance. While rust on exterior panels may detract from a vehicle’s cosmetic appeal, hidden rust can significantly compromise its structural integrity. If not discovered and addressed promptly, unseen rust can lead to safety risks and costly repairs.

As DIYers ourselves, we know that finding rust where it hides is crucial. Just look at what we discovered while tearing down our 1993 Range Rover Project in the pictures below. From door sills to floor pans, rust can creep into various crooks and corners, weakening your vehicle’s body structure. In this guide, we’ll explore common places to uncover these hidden rust spots and assess their impact on structural integrity. We’ll also share tips on how to repair the damage, ensuring your vehicle remains safe and roadworthy.

Wheel Wells

One often overlooked area where rust takes hold is the wheel wells. These hidden spaces behind your tires are prone to rust due to constant exposure to road debris and moisture. Rust in the rear wheel wells can compromise the vehicle’s structural integrity and allow water to enter the interior.

To check for rust here, thoroughly wash the underside of the vehicle including the wheel wells and surrounding areas. This is best performed with the wheels off the vehicle, supported on jack stands. Look for light rust in areas with where paint or undercoating are loose, flaky, or missing. When rust is immediately obvious, use a pick or screwdriver to determine the severity. If either of these tools perforates the sheet metal, plan to make more serious repairs as outlined below.

Rocker Panels/Sills

 Rocker panels and sills, running along the sides beneath the doors, are often overlooked until it’s too late. These panels are essential for maintaining the structural rigidity of your car, especially unibody cars. Their low position makes them highly susceptible to rust from road debris and moisture from the outside. But moisture trapped under the carpet and trim panels can weaken them from inside as well. Discovering rust here can be alarming, as it not only affects the appearance but also weakens the vehicle’s integrity.

Sills and rockers need to be examined from both inside and outside. To check for rust inside, remove any door sill trim plates and lift the carpet. Any dampness under the carpet should warrant deeper examination. On the outside, wash the bottom of the vehicle and look for signs of rust bubbles forming under the finish. Probe any visible rust with a pick or screwdriver to determine the severity and plan for repair.

Floor Pans

Floor pans live beneath carpets and insulation, which can hold moisture that silently corrodes the metal below without showing signs. These panels are essential to the structural integrity of the vehicle, ensuring safety and security for the occupants. Because they’re also exposed to harsh road debris, rust in the floor pans can spread quickly. If left unchecked, floor rot can go from a minor issue to a major structural problem.

As with rocker panels, floors need to be examined from both the inside and the outside to find rust. Clogged A/C condenser outlets or sunroof drains can let water collect under carpeting, as can missing vapor barriers inside doors. Damp carpets or musty interior odors are a sure sign to check the floor panels for moisture damage. From the outside, a visual inspection of the floors will typically reveal any rust damage. Probing with metal tools where rust is visible can help determine the extent of the damage.

Firewall/Cowl Panel

The firewall and cowl area, located at the base of the windshield, are often overlooked for rust. These areas can trap moisture when leaf debris prevents proper drainage of rainwater. Rust hidden here can become a point of entry for water behind the dashboard. This not only lets water into the interior, but it can also wreak havoc on the countless electrical connections and components that live under the dashboard. For DIY enthusiasts, addressing rust in the firewall and cowl area is essential for preventing extensive damage.

Finding rust in this area can be tricky, as the firewall tends to be crowded from both sides. The exterior of the firewall in generally pretty resilient to corrosion unless it is holding water unintentionally. From the outside, look for signs of rust bubbles forming, particularly at panel joints and drain locations. If you have sound or heat insulation mounted to the firewall, inspect behind it for any place moisture might be trapped. Inspecting from inside can be really difficult without removing the dashboard, carpeting and interior trim. However, should you locate a spot where moisture is entering the interior through the firewall, that’s the area to start inspecting for rust.

Trunk Floor and Spare Tire Well

The trunk floor and spare tire well present hidden rust hotspots in many vehicles. Often overlooked, this area can accumulate water and debris, leading to rust that can weaken the trunk’s structure. Addressing corrosion in this area is crucial for maintaining both the structural integrity and functionality of the trunk space.

While often neglected, this is typically one of the easiest areas to look for hidden rust. Leaking sunroof drains or poor trunk seals can let water into, so damp carpets or musty odors are often a clue to look for damage. Start by removing the spare tire and jack kit, as well as the trunk carpeting. This is generally held in place with clips or other small fasteners, if at all. As with all other exterior body panels, don’t forget to check from the outside as well. Stone chips and other minor damage underneath can lead to rust starting externally.

Fixing Hidden Rust

For minor rust, scuff or sand the affected area and apply a Rust Converter to chemically stop the oxidation process. Apply epoxy primer to the converted area to seal it, then repaint with the finish of your choice. Optionally, you can apply a product like Eastwood Rust Encapsulator as a single-step process once you’ve removed the loose rust.

For moderate rust such as small perforations, grind or cut out the affected area and weld in new metal. You may need to fabricate a small patch panel or use small pre-made weld-in plugs. Clean up the metal repair and treat the area with epoxy primer before applying paint.

For more extensive damage, such as severe perforation that has compromised the structural integrity, replace the entire panel. New replacement panels such as floor, rockers, wheel wells and other parts are available for most popular vehicles. These may be slightly more expensive than cutting a repair panel off an old donor vehicle, but entirely more reliable.

Pro tip #1 – Underside panels like wheel wells, rocker panels, and floors are exposed to rocks and other severe road debris. Consider using a rubberized undercoating as a topcoat to any repair made on these panels. This will provide greater resilience to stone chipping, minimizing future damage. The ultimate one-step solution is Rubberized Rust Encapsulator, which seals and protects in a single application.

Pro tip #2 – For rocker panels, sills, or any other enclosed panel, spray a coat of Internal Frame Coating to the inside surfaces after repair. This will prevent future rust from forming inside the panels.