Can I paint over rust without sanding it?

You can paint over rust without sanding it heavily, but you should at least remove loose rust flakes and dirt before painting, otherwise paint will fail quickly, as even rust-converting paints need some prep. Several products are marketed as direct-to-rust that claim no sanding required, such as Rust-Oleum Rust Reformer, which chemically converts rust into black paintable surface, POR-15 Rust Preventive Coating, and Corroseal Rust Converter, which can be applied directly over rust after removing loose flakes with wire brush. However, manufacturer instructions still say remove loose rust, scale, and dirt with wire brush or scraper, and clean with degreaser, which is minimal sanding but not zero prep. If you paint directly over thick flaking rust without any prep, paint will trap moisture underneath and rust will continue to expand, causing paint to peel within months. For light surface rust that is tightly adhered, you can often paint over without sanding to bare metal by using rusty metal primer first. For heavily rusted pitted metal, sanding or wire wheel to remove loose rust improves adhesion from 1 year to 5 to 10 years. So answer is yes you can paint over rust without extensive sanding to bare metal if you use rust converter or rusty metal primer and remove loose flakes, but completely skipping any prep and painting over loose rust is not recommended and leads to failure. Fix water source causing rust.