Having filled some cracks in the bathroom ceiling and allowed to dry, I used a bathroom emulsion to paint the ceiling. After a few days the emulsion cracked along the line of the original crack. Do you have any ideas why this should happen?

Pics help.

Did you rake out the crack first with something sharp? Just applying filler to a ceiling crack won't work unless you've done this.

Yes opened the crack first and then filled it. Looked fine until it was painted

What sort of filler did you use?

Could the moisture from the paint have re-activated / softened the filler?

Pretty sure it was Pollyfilla - mix powder with water type.

Is there movement in the ceiling - like barewood floors upstairs that get a lot of walking on...or under a kid's bed and said child jumps off a bunk bed every morning...or somenting like that. The bunk bed thing I've had twice....first time I came across it I was flummoxed for ages - Mum had lifted the carpets and there were bare boards on the kids room. Every morning wee boy leapt from bed and thumped full force on floor. I touched up and painted that ceiling about 4 times...then I had to line it and paint it again...only way... but she paid. I told her that she either stopped her kid doing that or papered the ceiling to get rid of the cracks. 2nd time I was quicker to check upstairs before I repeatedly filled and painted.

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You could try raking out the crack and using 2 Part wood filler whicch is much stronger than powder fillers. It is a bitch to sand though and needs at least two coats.