ᐅ Which type of filler should be used for the underside of a concrete balcony?

Hello,
since we can’t find anyone to do the finishing plaster on the underside and upstand of our concrete balcony, it’s up to me! (looking forward to it!)

Situation:
- Concrete balcony with an upstand, generally good surface and straight
- Balcony railing will be mounted on the vertical upstand
- Some pieces of binding wire (from the reinforcement) are visible on the underside and are already corroding

Planned preparation:
- Remove binding wire
- Concrete primer (PCI 303)
- Drip edge (Knauf p385t)

Questions:
- Is there a way to seal the rusting binding wire pieces? (e.g., epoxy with quartz sand) Otherwise, mechanical removal would be the only option
- Which filler (preferably fine-grained) is suitable for this work? Also considering compressive strength, as the balcony railing will be attached at the front edge (after plastering, according to the metalworker)
- It should be a filler that is easy to work with and can be applied smoothly. For larger areas, could one use quick-setting plaster profiles?

I would be very grateful for any answers.