Grind, Sand, Sandblast or Scour Off the Rust.
Or use chemicals to remove it or convert it to an inert form.
See: A rust converter versus rust remover.

Rust Converter:
Rust converters are a water-based chemical solution which contains two main active ingredients. These differ between products available commercially containing either phosphoric acid or tannic acid, or a combination of both. The other ingredient added is an organic polymer, which serves to provide a protective primer layer to the substrate it is applied to. Phosphoric acid converts iron oxide (rust) into an inert layer of iron phosphate, which is black in colour. This inert layer then acts as a barrier layer or protective coating.
You should still put a rustproofing coat on.
Products like:
Rust-oleum Rust Reformer.
Loctite Naval Jelly (phosphoric acid)
Or Dinitrol RC 900
The key benefit of Dinitrol RC 900 rust converter is that it penetrates rust 7 to 10 times deeper than standard rust converters. RC900 contains a complex organic chelat base and epoxy resin to actively convert the rust and is free of lead, zinc and chromium.

Rust Removal:
If you're looking to resurface metal and require that the end result be exposed rust-free bare metal, then a rust remover is better suited for the task.

Rust removal products contain oxalic acid or other corrosive and hazardous chemicals as the main ingredient. Sanding away as much of the rust as possible is advisable before the rust remover is applied. Surface preparation is key and a rust remover is applied to be absolutely sure that all traces of rust are removed before applying a rust proofing underbody or cavity wax.
It may require submersion of the part.
The rust removal can be pretty time consuming as the acid takes a while to be eaten away. There are a few on the market that claim to be non-toxic so be sure to look for those.

Rust Encapsulator:
This is a universal "paint over rust" base coat that seals in rust and stops it from spreading. It flows into hard-to-reach areas, penetrates deep into the rust, and fills in pinholes and surface imperfections. It can be applied to any surface, even partially rusted ones. It is the last step in the rust neutralization/removal work before starting with primer, paint, and the rest of the finishing process.

See See Removing Rusted Bolts
How To Remove Rust from Metal | FamilyHandyman
5 Tried and True Methods For Removing Rust From Metal Objects | ApartmentTherapy

See: Key Differences - Rust converter versus rust remover | REJEL
Rust converter versus rust remover What's the Difference | azrust
See Removing Rusted Bolts

See Lubricants

Safety Data Sheets (Contents of Rust Converters):
Loctite Navel Jelly Saftey Data Sheet
Hazardous Component(s) CAS Number Percentage*
Phosphoric acid 7664-38-2 10 - 30
2-Propanol 67-63-0 1 - 5
Xanthan gum 11138-66-2 1 - 5
Sulfuric acid 7664-93-9 0.1 - 1
Silica, amorphous, fumed, crystal-free 112945-52-5 0.1 - 1
Diiron trioxide 1309-37-1 0.1 - 1
Magnesium oxide 1309-48-4 0.1 - 1
Aluminium oxide 1344-28-1 0.1 - 1
Alcohols, C11-14-iso-, C13 rich, EO~ 78330-21-9 0.1 - 1

Rust-oleum Rust Reformer Aersol Safety Data Sheet Feb 2022
Main Ingredients Vinyl Acrylic Resin 25-50%, Barium Sulfate 2.5-10%
Chemical Name CAS-No. Wt.% GHS Symbols GHS Statements
Acetone 67-64-1 25-50 GHS02-GHS07 H225-319-332-336
Propane 74-6 10-25 GHS04 H280
n-Butyl Acetate 123-86-4 10-25 GHS02-GHS07 H226-336
n-Butane 106-97-8 2.5-10 GHS04 H280
Hydrous Magnesium Silicate 14807-96-6 2.5-10 Not Available Not Available
Xylenes (o-, m-, p- Isomers) 1330-20-7 2.5-10 GHS02-GHS07 H226-315-319-332
Dimethyl Carbonate 616-38-6 2.5-10 GHS02-GHS06 H225-331
Hydrotreated Light Distillate 64742-47-8 1.0-2.5 GHS08 H304
Carbon Black 1333-86-4 0.1-1.0 Not Available Not Available
Ethylbenzene -41-4 0.1-1.0 GHS02-GHS07-GHS08 H225-304-332-351-373
Naphtha (Petroleum), Heavy Aromatic 64742-94-5 0.1-1.0 GHS07-GHS08 H304-332
Solvent Naphtha,Light Aromatic 64742-95-6 0.1-1.0 GHS07-GHS08 H304-332
Zinc Phosphate 7779-90-0 0.1-1.0 Not Available Not Available
Sulfonic Acids, petroleum, calcium salts, overbased 68783-96-0 0.1-1.0 GHS06 H330
Zinc Oxide 1314-13-2 0.1-1.0 Not Available Not Available
Methyl Ethyl Ketoxime 96-29-7 0.1-1.0 GHS05-GHS07-GHS08 H302-312-315-317-318-331-336-350-370-373

last updated 1 Nov 2022