Q: Extremely Rusty bottom

Although the bottom of the truck being rusty may not immediately be a cause for concern, over time, as you may know rust operates a lot like a cancer and will tend to spread rapidly eventually causing parts to break or become brittle and fall apart. There are a number of ways to remove rust and prevent it from coming back. Rust is a chemical reaction between iron, H2O (water) and oxygen resulting in iron oxide. You can physically remove rust by sanding and scraping it off with sand paper. Chemically removing it can be done involving phosphoric acid, however this can be dangerous as it can cause harm to human tissue if not applied properly. Encapsulating or sealing the rust can be done by preventing oxygen from getting to it any further. These typically come in the form of a filler or paint like application.