What happens if you just paint over rust?

What happens if you just paint over rust without proper preparation is that rust continues to grow underneath paint, causing paint to bubble, peel, flake, and fail within months, and corrosion spreads, making metal weaker. Rust is iron oxide formed when iron reacts with oxygen and moisture, and it is porous and holds moisture, so if you paint over loose rust, you trap moisture and oxygen under paint film, allowing rust to continue expanding underneath. Initially paint may look okay, but within weeks to months, rust expands, pushing paint off as blisters and bubbles, exposing more rust, and corrosion continues deeper into metal. This results in wasted time and paint, as you will need to strip and redo job. For light surface rust that is tightly adhered, you can paint over it if you first wire brush to remove loose rust, sand to smooth, clean, and apply rust converter primer like Rust-Oleum Rust Reformer that chemically converts iron oxide into stable black coating, then topcoat with Rust-Oleum Stops Rust. For heavy flaky rust, you must remove all loose rust down to bare metal or stable rust before priming. Simply painting over rust with regular latex house paint offers almost no protection and will peel quickly. Using rust-inhibitive paint helps but not without prep. So proper steps are remove loose rust, apply rust converter or rusty metal primer, then topcoat for longest protection of 3 to 5 years.