How to Fix Rust on a Car

No matter how well you maintain a vehicle, rust can pop up on body panels, under the hood, or throughout the chassis—especially if you live in a northern or coastal climate. And once corrosion begins, it doesn’t stop on its own. Rust spreads and multiplies like a virus, weakening metal until what was once merely an aesthetic issue becomes a serious structural problem. 

Whether you’ve noticed bubbling paint on a fender, flaky orange metal underneath your doors, or surface corrosion on the frame or suspension, understanding how rust forms and how to properly repair it can help to stave off corrosion and extend the life of your vehicle. In this guide, we’re breaking down what rust is, how to fix it, and how to prevent it from returning once repaired. 

Rust, also known as iron oxide, is a form of corrosion that occurs when steel or other ferrous metals are exposed to oxygen and moisture over time. The chemical reaction causes the metal to oxidize, resulting in the crusty, reddish-brown material commonly seen on weathered vehicles and uncoated metals. 

Unlike dirt or surface staining, rust consumes metal—it isn’t just aesthetic. As corrosion progresses, it weakens panels, compromises structural integrity, and eventually leads to holes, cracks, and the destruction of affected panels if left untreated. 

Rust doesn’t appear overnight—at least, nothing deeper than surface level. This corrosion occurs after repeated exposure to the elements, often following the failure of protective platings and coatings. Some of the most common causes and exacerbating factors are: 

• Excessive moisture: Rain, snow, high humidity, and standing water can accelerate corrosion, leading to rapid rust formation in certain climates. 

• Road salt: Salt is an effective deicing agent used in winter climates to prevent roads from freezing. However, acting as an electrolyte, this compound dramatically accelerates corrosion. 

• Bare metal exposure: So long as rust-prone metals are adequately coated, they won’t rust. However, when exposed to the elements, moisture from scratches and paint chips can reach the raw steel and quickly corrode it. 

• Poor drainage: Without proper drainage through drain holes in the bed, doors, or rocker panels, stagnant water can build up, rotting panels from the inside out. 

• Mud and debris buildup: Dirt, road salt, and grime cling to the body and chassis, especially in wheel wells and undercarriages. These areas are often the first to develop rust. 

Over time, factory rust protection fails—from undercoating and cavity wax to paint and plating. Once these coatings fail, it’s only a matter of time before rust develops, especially on vehicles regularly exposed to harsh conditions.

Not all rust is a deal-breaker. Often, corrosion is merely cosmetic—however, once structural damage sets in, a vehicle may no longer be safe to drive. 

Surface rust, like small bubbles beneath paint and light scaling, is typically repairable and not immediately dangerous. Structural rust, on the other hand, can become a serious concern if it reaches vital components like frame rails, suspension, brake lines, and rocker panels. If metal becomes soft, perforated, and flaky, repairs can be expensive and often dangerous to ignore. 

As a general rule, surface rust is manageable, while structural rust should be taken seriously and repaired ASAP. If you’re unsure which type of rust your vehicle has, consider getting a professional inspection.

Rust repair costs can vary widely depending on the severity and location of the corrosion. In some cases, quick DIY surface repair can cost as little as $50 to $200 in materials. Professional repairs for the same amount of work can reach up to 10x that amount, while heavy structural repairs can easily reach the tens of thousands. 

Early intervention is key to repairing rust. Catching rust before it turns into deep corrosion can save thousands in repair bills.

Surface rust and deep rust, while similar in appearance, require very different repair approaches. Check out the table below for more insight into these two rust types. 

Surface rust occurs when the paint or protective coating fails, exposing bare metal. While aesthetically displeasing, the corrosion hasn’t yet become serious. Instead, it remains on the panel's outer layer and hasn’t compromised the metal's structural integrity. 

Surface rust typically presents in a few characteristic ways, including minor paint bubbling, orange or brown discoloration, and light scaling without serious flakes or holes. Once the surface rust is sanded down, solid metal remains.

If caught before it has time to spread excessively to transform into deep rust, surface rust repairs can be very DIY-friendly. The following step-by-step guide details how to perform the repair.

All rust and failing paint must be completely removed before continuing the repair process. Use 80–120 grit to remove rust, and 180–220 grit to smooth edges and remove deeper scratches from the surrounding paint and primer. Make sure to get all the rust, as any left behind will continue to spread beneath the new paint. 

Before applying primer or paint, clean the panel thoroughly with dish soap and water, then use a solvent to remove grease and residue. Make sure the panel is completely dry before moving on to the next step, as trapped moisture can sabotage the repair. 

Rust converters chemically neutralize any remaining oxidation for a solid, rust-free paint base. Apply the converter according to the manufacturer's instructions for best results.  

Before applying paint, you’ll need to prime any bare metal to ensure a proper bond. Use an epoxy primer or self-etching primer to seal the metal and prevent future corrosion. This step is essential for longevity—otherwise the paint will likely fail and reintroduce moisture to the bare metal. 

Light pitting may remain after applying the primer, requiring filler or glazing putty to repair. Apply a thin layer of high-quality body filler to the area and sand it smooth once dry. Prime once more, and prep the area for base and clear coats. 

Deep rust occurs when corrosion doesn’t just sit on top of the bare panel, but eats through the metal itself. At this stage, the material loses strength and cannot be reliably repaired with a rust converter and filler alone. Deep rust typically requires the damaged metal to be cut out and replaced with new, solid metal, which is then welded in place.

Deep rust is easily identifiable. Common characteristics of this type of rust include visible holes and cracks in panels, flaking or crumbling metal, soft spots that can easily be flexed or broken, and rust that spreads from the inside of the panel outwards (rust bubbles). 

Deep rust repair is typically more involved and is better left to professionals. Failing to repair deep rust correctly almost always results in the rust returning within a year or less. 

As a first step, you’ll need to clear away all flaking rust, paint, and filler from previous repairs. Using a wire wheel and sander, expose the full extent of the damage and reveal any hidden corrosion beyond what’s immediately visible. 

Filling rust holes is never a permanent solution. The only way to prevent rust from returning to the affected areas is to remove and replace all compromised metal. Do so using your choice of methods, including a grinder and cutoff wheel, spot weld cutter, band file, etc. 

Patch panels can be sourced from OEM suppliers, aftermarket manufacturers, or donor vehicles. If you’re inexperienced with welding, a body shop is strongly recommended to prevent burning holes through body panels and ensure a strong repair. 

After the new patch panel is installed, grind the welds smooth to eliminate any high spots. Then, apply filler as needed to fill any low spots. Once sanded smooth, apply a primer coat to seal the bare metal. For best results, have the repaired area professionally painted and blended for a seamless repair. 

The best way to repair rust is to prevent it from forming in the first place. Follow these proactive steps to dramatically extend your vehicle’s lifespan:

• Touch up rock chips and scratches promptly

• Wash the undercarriage regularly, especially after winter driving

• Keep drain holes clear in doors and beds

• Apply rust inhibitors or undercoating

• Install running boards to protect rocker panels

• Use hood protectors to reduce paint chips

Truck owners can also benefit from accessories that limit moisture exposure, such as mud flaps, side window deflectors, and more. 

Whether you should or shouldn’t buy a vehicle with underbody rust depends entirely on the level of severity. Light surface rust and corrosion are common and typically manageable. However, structural rust can be dangerous and expensive to repair. 

Always inspect areas like the frame rails, suspension mounting points, brake lines, and fuel lines for extensive corrosion before agreeing to purchase the vehicle. When in doubt, have the vehicle inspected before purchase or walk away from the purchase. 

Completely preventing rust can be next to impossible, depending on where you live and whether or not you drive your vehicle in the winter. However, slowing its formation and reducing severity can be accomplished with parts and accessories from RealTruck. Products like tonneau covers and bed liners keep moisture out of truck beds, while mud flaps and running boards help to prevent rocker panel paint chips and caked-on mud that could exacerbate corrosion. 

With decades of experience serving truck owners, RealTruck helps you protect your investment and prevent rust from taking hold.