Filled crack showing through paint

[DIY67]

I have filled a ceiling crack by correctly scraping along it with a sharp tool before filling it with Poly filler from a tube. It says it doesn't shrink etc. I then sanded it to a perfect smooth surface and painted with white paint. So why oh why oh why after several coats is a dark line still showing? This doesn't make sense.

 

[sparkwright]

Did you dilute the first coat of paint?

How many coats of paint have you applied to the crack?

So it's filled perfectly? Not too proud? Not too shallow?

What paint have you used?

 

[Wayners]

Put straight edge across. Probably a shadow as not flat

 

[RandomGrinch]

A possibility I've come across - if you've sanded down into the edge of the plasterboard, without a sealing coat; the paint can cause the paper edge of the plasterboard to swell slightly.
This can create a ridge in what was a perfectly level finish.
If this is the case, sand back and fill again.

 

[Wayners]

Try mixing a little powdered filler with paint to thicken.
4 inch roll over crack.
Let it dry.
Paint.

 

[DIY67]

I gave the whole plastered ceiling a watered down base coat and then filled. Def flat as sanded flush with the ceiling but like you said, it could have risen. I'll try the powder mixed with paint thanks.