I have an apartment where previous owners have done a lot of holes on the concrete ceiling for very long a curtain rail. They removed the curtain and the plugs and sealed the holes with single no more nails (very soft material, easy to remove. They did not filled the holes deep).
I want to full restore the ceiling so a new type of curtain can be installed drilling in any zone and installing new plugs/anchors withouth the risk of the plug to be loose. How can I fully restore the ceiling?
I've done this very same question to many constructors and I have basically these possible answers.
1.- Clean the holes of dust and any material and fill the holes with epoxy (a product like sikadur 31). Epoxy is harder than concrete and it will support redrilling.
2.- Clean the holes of dust and any material, apply concrete bonding agent (a product like sikadur 32) then apply repair mortar based on hydraulic expansive cement. Concrete bonding agent would help to create a monolithic between the original concrete and the repair. Redrilling would be possible after a long dry-cure period (weeks).
3.- Opposite to #1: even though epoxy would support redrilling the drill tip would tend to drift to the softer area which is concrete, so you won't be able to drill in the same spot again. Solution like #2 is better.
4.- Opposite to #2: the repair zone would fail on a redrill attempt. Two posibilities: a) the whole repair patch would chip out b) it would be possible redrill and put a new plug/anchor at first, but eventually with some weight load (like a curtain) the plug would strip out of the hole. Solution #1 is better.
5.- Worst of all: there is not possible solution at all (except redrill a bigger holes where possible). This option does not work for me.
Curiosly constructors supporting options #1 & 2 have told me that they have tried these solutions with success.