Paint turning to powder

[Devil]

I have recently repaired some paint work for a customer in a kitchen, (external wall) with their original paint that was peeling but in only three weeks it's all off again and very powdery and looking like wet patches? The walk isn't damp at all and the outside wall is sparred with no damage or place for water to penitrate ???

I have used the same paint on another patch which is looking perfect ??
Maybe salts coming out of the plaster ?
If so how can I get over the problem,

Thanks for your help !!!

 

[dcdec]

For efflourescence to come through there would have to be some damp in the wall. It could be that it has now dried following an earlier problem. You could try alkali resisting primer. Dry brush any powder residue off first.

Other than that i don't know, pictures and more info is needed