Os-Pho. A surface prep product that converts a rusted surface to iron phosphate, that gives inherent rust resistance and "tooth" for paint.
https://www.amazon.com/gp/product/B003Q7XWU4/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1
Heavy rust can be removed first by wire brush or needle scaler.