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.

I would remove any rusted parts as much as possible and, if necessary, apply a rust protection primer. If you are already using a bonding primer from PCI, you could also use a filler compound like PCI’s Polycret. However, this is not necessarily required when using the bonding primer. But I can’t really judge that from a distance.

If you want to install a fabric corner bead, I would prefer a bonding reinforcing mortar. In that case, it’s no longer just simple filling for me. I would also avoid using quick-set plaster profiles. Or is the visual requirement that high? This approach would create a lot more work for you since the profiles need to be removed afterwards. If the visual standard is that demanding, you could use Paris moldings instead—a kind of “quick-set plaster rail” made from mortar.
What exactly do you mean by very fine? I rarely work with Polycret, but I find it fine. A bonding reinforcing mortar that can also be applied as topcoat plaster usually has a 1 mm (0.04 inch) grain size, which feels coarse to me. Outside walls are usually not filled as finely as interiors. A fine concrete filler can actually be well rubbed smooth with a fine sponge float, and for my standards, that looks very fine already. The easy-to-apply, low-shrinkage filler compounds are generally not very resistant to pressure. The question is whether it might be enough to just install the corner bead and then precisely smooth the surfaces with filler. Of course, that won’t result in a perfectly straight and even surface.

Thank you, jcan, for your quick and detailed response.
Despite researching, I’m still unsure about the correct approach.

I would like to follow your advice but have a few questions:

- Today I started mechanically removing the leftover binding wire pieces. It’s a lot of work since the balcony is 16m (52 feet) long and there are many pieces. Another issue is that wire remnants are sometimes hidden 2, 3, or 4 mm (0.08, 0.12, 0.16 inches) deep, and after two years they still haven’t rusted, but I’m worried they might eventually start rusting. Is there a rust protection treatment that can be applied over the entire surface in addition to removal?
- The upstand is about 35 cm (14 inches) high, and a cover plate will be installed on top. Is a drip edge absolutely necessary there? My idea was to use it as a stop as well.
- Regarding the finish... I just don’t want a rough plaster. Next to it, there is a boxed area that was very finely skimmed. It should somewhat match but this is not that important. Much more important is a good and easy application, since although I have done a lot myself, I’m not really confident with this and actually planned to hire a painter who has now backed out.
- I need to take a closer look at the edge profile. The underside could be scraped sharply if the profile allows it. Also, the two surfaces don’t have to be skimmed with the same material. Only the upstand needs to be pressure-resistant and perfectly vertical.

“Pariser Leiste” … amazing … never heard of it before and so simple and logical!!!

The rust protection I was referring to is just a rust-inhibitive paint. I would use it only selectively where the metal is exposed. Of course, there are many other options for concrete repair, but I can’t say if those are necessary in your case. Based on experience, it usually takes a long time for corrosion to develop through.

I probably wouldn’t apply an additional drip edge in plaster if the metal flashing has a proper 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 only apply the concrete fillers very thinly. If there are many holes, some preparatory work will be required. Whether a corner bead is needed for aesthetic reasons or because the vertical line is uneven is your decision. Without photos, it’s difficult to make a clear recommendation.

Your answers have been very helpful, thank you very much!

I will now proceed without a drip edge and apply the concrete filler only very thinly, since the surfaces are already quite even and straight.

Could you recommend a compressive strength concrete filler for the balcony railing? Is 10 N/mm² (1450 psi) sufficient for this? Regular plaster usually only has up to 6 N/mm² (870 psi). I came across Ar..x B12 (or PC. Po...cret 5).

I also have a question about substrate preparation: since the underside formed by the shuttering boards is perfectly smooth (not glossy, but matte), do I still need to prime it?

Thank you very much, jcan, for your professional advice. This is my last question, I promise!!!

That is already very pressure-resistant. Considering the expected layer thickness, it doesn’t make much difference anyway. So it should work well for you.
The B12 adheres very well to a clean substrate. An additional primer is usually not necessary.