Surface rust and small bubbles are DIY-fixable if you grind to clean metal; rust that's eaten through needs a patch panel or a pro.

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Tools I used

Tool	Why you need it	Link
Angle grinder + 80-grit flap discs	Takes rust back to bare metal fast	Check price →
Wire wheel attachment	Cleans pits and tight curves	Check price →
Rust converter	Neutralizes rust left in pits	Check price →
Epoxy primer	Seals bare metal so rust can't return	Check price →
Body filler + spreaders	Rebuilds the shape	Check price →
Sanding block + 80/180/320 paper	Shapes and smooths the filler	Check price →
Seam sealer	Seals the arch lip from water	Check price →
Safety glasses, respirator, gloves	Grinding throws sparks and dust	Check price →

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Steps

Mask a wide area. Remove the wheel for easy access to the arch lip.
Grind every bubble back to shiny metal, going 2–3 cm past the visible rust.
Poke the metal. If a screwdriver goes through, stop here and plan a patch panel.
Wire-brush the pits, then apply rust converter and let it cure.
Spray epoxy primer over all bare metal. Let it cure fully.
Apply thin layers of body filler. Shape with 80 grit, then smooth with 180.
Seam-seal the inner lip of the arch.
Prime, wet sand with 320, then base coat and clear to match.

Rust you can see is always smaller than the rust you can't. Keep grinding until it's clean.
Never put filler straight on bare or rusty metal. Primer first, always.
Rinse salt out of your arches every spring. Road salt is brutal on wheel lips.