Which spatula to use for balcony underside (concrete)?

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

Situation:
- concrete balcony with upstand, basically good surface and straight
- the balcony railing is mounted on the vertical upstand
- partly pieces of binding wire (from reinforcement) in the underside, already corroding

planned preparation:
- remove binding wire
- concrete bonding 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 only mechanical removal remains
- which filler (preferably fine-grained) is suitable for this work? Also with regard to compressive strength, since the balcony railing will be attached to the end face. (after plastering, as the metalworker said)
- it should be a filler that is easy to apply and smooth to spread. Could one work with rapid plaster profiles for a larger area??

I would be very glad to receive answers.

I would remove rusting parts as far as possible and, if necessary, prime them with rust protection. If a bonding primer from PCI is already being used, you can also use a filler like Polycret from PCI. Although this is not necessarily required with the bonding primer. But I can't assess that from a distance.

If a fabric corner angle is to be processed, I would rather use an adhesive reinforcing mortar. For me, that is no longer just filling. I also wouldn't use quick plaster profiles. Or is the aesthetic requirement that high? That means much more work for you. The profiles should be removed again. If the requirement is that high, you can also use Paris moldings. So a kind of "quick plaster rail" made of mortar.
What is meant by "as fine as possible"? I rarely work with Polycret, but it is fine. An adhesive reinforcing mortar, which can also be applied as a top coat, has a 1 mm grain and seems coarse to me. Outside, you don’t plaster as finely as inside. A fine concrete filler can actually be well smoothed with a fine sponge float, and to my standards, that already looks very fine. The easy-to-process, low-stress fillers, however, are not so pressure-resistant. The question is whether it is enough to insert the corner bead and then smooth the surfaces sharply with the filler. Of course, it will not be "perfectly" straight and even then.

Thank you jcan, for your quick and detailed answer. Despite researching, I am simply not clear about the correct procedure.

I would like to take your advice, but I still have questions:

- today I started mechanically removing the leftover binding wire pieces, very much work, the balcony is 16m long and there are many pieces. In addition, there is the problem that wire residues are partially hidden 2, 3, 4 mm deep, which have not rusted after two years, but I fear they will start rusting eventually. Is there a rust protection that can be applied fully after removal?
- the upstand is about 35 cm high, and a cover sheet will be placed on top. Does it absolutely NEED a drip edge? My idea was to also use it as a stop.
- regarding the fineness... it just shouldn’t be rough plaster. Next to it is a boxed-in area that has been very finely puttied. It should somewhat match, but it’s not so important. Much more important is a good and simple application, because although I have done a lot myself, I don’t really trust myself with this and actually wanted to have a painter do it, who has now dropped out.
- I need to take a closer look at the edge profile, the underside could be scraped sharply if the profile allows. Also, the two surfaces do not have to be filled with the same material. Only the upstand must be pressure-resistant and perfectly vertical.

"Pariser Leiste" ... amazing ... never heard of it before and so simple and logical!!!

The rust protection I meant is just a rust protection paint. I would only use it spot-wise where the metal is. Of course, there are many other options in concrete repair, but I can't tell you if that's really necessary. In my experience, it doesn't rust through that quickly. I probably wouldn't plaster an additional drip edge if the cover sheet has a reasonable overhang. What you apply exactly also depends somewhat on the condition of the concrete. In some cases, the formwork was done so well that you can really only work thinly with the concrete fillers. If there are many holes, preparatory work is necessary. Whether a corner profile is needed for aesthetic reasons or because the vertical is crooked is again your decision. Without photos, making a recommendation is difficult.

Your answers have helped me a lot, thank you very much!

I will now work without a drip edge and apply concrete filler only very thinly, as the surfaces are already relatively even and straight.

Could you recommend a pressure-resistant concrete filler (because of the balcony railing)? Is 10N/mm2 sufficient for this? Normal plaster also only has up to 6N. I ended up with Ar..x B12. (or PC. Po...cret 5)

And I still have a question about substrate preparation, since the underside through the formwork boards is mirror-smooth (not shiny, is matte), I have to prime after all, right?

Many thanks jcan, for your professional tips. This is also my last question, promised!!!

that is already very pressure-resistant. With the expected layer thickness, however, that doesn't matter so much. So it should work well for you. The B12 adheres very well to a clean surface. An additional primer is usually not necessary.