Filler keeps cracking

[iamchris]

Hi all, I had a large crack in the wall which I think was caused by damp around the window. Anyway I filled it and and painted over and it looked great. Week or so later and I've noticed hairline cracks form on the join of the filler / crack. Slapped a bit more filler over the top and gave it a light sand and left it. Few days and and I now notice hairline crack forming again.

What's the deal here? Do I need to sand it back, tape the crack which I didn't do before. Fill and sand again? Or do I just need to use a different filler? This is the one I have been using in the link below. On thing also is this room is south facing and the sun hits right where the filler is. So I don't know if maybe the sun drying it out is doing it?

https://www.screwfix.com/p/no-nonsense-interior-ready-mixed-filler-white-off-white-1kg/555gt

 

[Alastairreid]

You will need flexible filler, cut it back a few inches tape then fill.

 

[bobasd]

iamchris,
you mention damp as a possible factor?
If thats the case then the above advice is useless.
You have to find the cause of any damp and the "hairline cracks".
Damp can spread under the repair and surrounding plaster, and swelling will keep blowing off any badly thought out repairs.

Before doing anything else why not post pics showing inside the window reveals, and the opened up crack - and pics of outside where the frame is sealed?

 

[iamchris]

Hi, sorry I should have mentioned the source of the damp was repaired a while ago. There was a gap around the window frame, I have since had the windows replaced and the gap sealed.

Which flexible filler do people recommend?