Degrease the item first.
Rotary wire-brushes will evict the worst paint and rust. For paint that’s still fully hard, use paints-stripper first but wash it all off before the brushing!
Citric acid solution is quite a good rust dissolver, as I found on a very rusty tap, but obviously it won’t fill in pits where the iron has gone back to Nature.
Then ‘Scotchbrite’ or similar, or wire-wool (wear suitable gloves).