Look for rust on the frame rails that pass directly under the doors. Also check for rust inside the wheel openings and along the exhaust system. Uneven lines in the paint often mean that the car was damaged and repainted. If the rust is only superficial, it's a used vehicle you can keep.
But if it affects the structure, it can be a recipe for pain. Review the vehicle's history and ask a mechanic to check areas where you can't. If you think a car has been repainted due to rust, be sure to look at it in the sunlight to get the best possible view. Look for “bubbles” in the paint.
This is a common problem when a car is repainted, but the underlying rust has not been removed. Think about how easy it is to crumble a layer of rust between your fingers, and then imagine that thing trying to protect you and your loved ones during a car accident. This is called embedded rust, and if it continues, this penetrating rust corrodes metal, creating holes and causing body panels to fall apart. It's not uncommon for some components, such as the muffler and exhaust system, to be rusted, and that's not necessarily a reason not to buy the car.
It's very common for a car body to rust a little, and it's no big deal if it's detected early and repaired. Stone chips and other notches and dents that don't repair usually turn into rust spots over time, so it's worth buying some matching paint to cover these imperfections. Door panels and rocker arms (the panel that extends below and along the front and back of the car's front and rear doors) are often rusted. Most modern cars are designed with extensive anti-corrosion measures, including transparent paint finishes that protect both the paint and the body panels and galvanized coatings that protect the steel structure of the vehicle body.
Touch-up paint can be applied to stone fragments, small scratches, and other minor notches and dents, but repairing rust requires several steps, several tools and materials, and a little skill. Many car wash products use an underbody spray that can help remove salt, dirt, and grime from critical areas that aren't easily seen, such as the chassis, suspension components, and wheel gaps. The drain holes are located at the bottom of the doors and in the tilting panels (the area below the doors); by maintaining them, water will not accumulate or rust.