Since you want to touch-up only (and are willing to live with that splotchy look) Here's what I'd do: Get some SEM Rust Mort (it's a blend of phosphoric acid and "stuff") and some small detail brushes. Clean the whole frame with a solvent wash (denatured alcohol or acetone for example, wear gloves). Apply the Rust Mort with detail brush to all the exposed rust, you'll need to keep applying, several times, to keep it wet until it consumes and converts the rust to iron phosphate. Where you have those little bubbles of rust underneath paint you have to decide whether to break the blisters (I would) and apply the acid or leave them unbroken.
I doubt you'll find nail polish in that exact shade of blue, so if you can't then get model paints you can mix to achieve a match. Dry off the acid treated spots with a clean cloth, paint over the spots and scratches and overlap edges and apply as many coats needed to get complete coverage. After the paint has cured (wait 2 weeks for certainty) wax the entire (cleaned) frame with high quality automotive wax (I use Mother's Pure Carnuba with NO cleaners; cleaners are abrasive). Keep it waxed, apply periodically.