The best way is to use a concentrated rust remover or gel.
Ive used this on my bug and it works a treat. The stuff i used was made by a company called Bilt-Hamber in essex. Its expensive but its worth it as it works. They also sell a good wax oil called dynax s50 ive just coated the inside of my heater channels with it through the running board bolt holes.
It looks like it's mostly just surface rust, (You lucky dog).
Ideally it would get sandblasted back to clean steel, primed and painted, but a good alternative is to wire brush (on a drill), to remove flaky stuff, then paint with a good rust sealing product. POR15 from Frost gets rave reviews, and there's always the Hammerite range. Beware of painting over the trackrod threaded ends and locknuts, they are tight anyway, even tighter if they've rusted a bit, and paint on top makes tham very awkward to undo later. Have fun.
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