Apartment built 88-90. Concrete ceiling that was sand-textured, so I sanded it down. "Filled" with Nordsjö medium filler and sanded down. No bubbles. Then I chose to reapply with Alcro roll filler since I was redoing the walls and had slightly better tools. There were bubbles here and there, quite a few in places, but not everywhere. The walls that are made of concrete (which received the same "treatment" as the ceiling before) do not show the same symptoms.
What could be causing this?
