# Car rustproofing: when it is needed and what affects the price _Updated: 8 November 2026_ **Short answer:** Rustproofing is wax in the hollow parts of the body and a coating on the underbody, the underside of the car. An older car needs it, a car after an accident, and a car whose old coating has cracked, because that is when salt and water reach the metal. The price depends on what is protected and how much rust there is before coating. ## Salt and moisture eat the metal from below Rust forms when water and air reach the metal. Road salt speeds it up. For the state roads alone, Serbia's road company Putevi Srbije stocked 75,000 tonnes of salt last winter. Stones and age wear away the paint and protective layer, leaving the metal exposed. Driving on gravel roads and through puddles speeds rust up even more. Under the car, rust is noticed last. It often starts on the inner side of the metal and only later shows on the paint. Rust on the underbody weakens the load-bearing parts, so the car becomes less solid. It can also lead to brake fluid leaks. ## When a car needs rustproofing - **The car is 6 to 8 years old or more:** Factory underbody protection wears off over time. It is renewed after 6 to 8 years at the latest, and sooner on a car that often drives on salted roads in winter or on gravel. - **The car has been in an accident or had bodywork repairs:** Where the protection is damaged or the metal was left without it, rust starts. - **The old underbody coating has cracked, gone brittle or is falling off:** Water gets in under it. Brown spots on the coating mean rust has already started underneath. - **A bubble under the paint:** Water and air have got between the paint and the metal, and rust will soon break through. - **A wet carpet in the cabin or boot:** The metal under the carpet has usually already rusted. ## Where rust most often starts - **The sills and frame rails under the doors**, on both sides of the car. - **The wheel arches**, especially on older cars without plastic liners. - **The exhaust**, because moisture attacks it from inside and outside, and mud holds it there. - **Suspension parts**, because the wheels throw mud and water at them. - **The boot floor**, under the carpet. - **The metal around the windows**, because water easily leaks in there. - **The doors**, outside and inside. ## How rust progresses and when it fails the roadworthiness test - **A brown spot on a scratch or dent:** The rust is only on the surface. This is when it is easiest to stop. - **The metal under the paint is flaking:** The rust has got into the metal, and salt eats it even faster there. - **A hole in the metal:** The part probably has to be replaced. Surface rust does not fail the roadworthiness test in Serbia. The Rulebook on Vehicle Roadworthiness Testing lists as a defect deep rust on the load-bearing parts of the body, the chassis and the floor, and rust that has made a hole. With such rust the car is not certified until it is repaired. Protection done in time keeps exactly those parts safe. ## How rustproofing is done Before protecting the car, we remove the plastic covers, wash and fully dry the car and take off flaking rust. We spray following the drawing for your model, so as not to damage electrical parts and airbags. First the wax goes into the cavities, then the protective coating onto the underbody. ## Wax or bitumen for the underbody - **Wax:** It is transparent. It stays soft and creeps into corners and joints, so it keeps moisture out well. It handles stone chips less well. - **Bitumen:** A thick coating made from oil, not transparent. It is hard and handles stones and gravel roads better. Over time it cracks, water gets in under the coating through the cracks, and the metal underneath rusts. ## What the price of rustproofing depends on - **Underbody only, or cavities too.** Cavity protection is a separate job. - **How much rust has to be removed.** The more rust, the more work before coating. - **The old coating.** When it has cracked, we remove it before the new one, and that takes a lot of work. - **The whole floor or just part of it.** Sometimes we renew only the damaged area. - **A rust hole.** Then we repair the metal first, and only then do the protection. ## How to make the protection last longer - **In winter, wash the car once a week, underneath too,** because salt stays under the car. At a car wash choose the underbody wash, or wash the underside yourself with a pressure washer. [More about preparing your car for winter](https://autoindustrija.rs/en/advice/winter-car-checklist) - **An underbody wash does not replace protection.** It removes the salt, but after the next drive on a salted road the metal is exposed again. - **Scratches and dents:** fix them in time, because rust starts on them. - **Garage:** park the car in one when you can. - **Look at the underbody** every time the car is up on a lift: look for cracked coating and brown spots on it. ## Rust on a used car before you buy A patch of paint that differs from the rest of the car may be repaired rust. If the repair was not done well, the rust comes back. Surface rust is easy to fix. Rust that has gone deep into load-bearing parts means a major repair and a failed roadworthiness test. When you find rust, ask the seller to lower the price of the car by the cost of the repair. [More about checking a used car](https://autoindustrija.rs/en/advice/used-car-stolen-check) ## Frequently asked questions ### Does a new car need rustproofing? A new car has factory rust protection. Extra protection is needed when the factory protection is damaged: after an accident, after bodywork repairs and when the underbody coating cracks. ### Can rustproofing go over rust? No. Rust under the coating keeps eating the metal and can eat through it even under new protection. That is why before coating we remove the rust and any old coating that has cracked, because moisture collects under it. ### Does car wax protect against rust? Car wax protects the paint. The metal from inside is protected by the wax we inject into the cavities, and from below by the underbody coating. Have a question about your car? Message us on WhatsApp or call 067 774 1606. ## Read also - [Getting your car ready for winter: 8 checks](https://autoindustrija.rs/en/advice/winter-car-checklist) - [Car battery in winter: why the car won't start and how to protect it](https://autoindustrija.rs/en/advice/car-battery-in-winter) - [Car battery: which one to choose and what to look out for](https://autoindustrija.rs/en/advice/which-car-battery) --- Page on the website: https://autoindustrija.rs/en/advice/car-rustproofing auto-industrija, Belgrade. Phone and WhatsApp: +381677741606. E-mail: kontakt@autoindustrija.rs.