Mortar joints appear damp?

[Tadders]

Hello everyone.

I hope someone might be able to explain this strange pattern that has appeared on my neighbors conservatory wall. It appears that the mortar joints are damper than the block work. This has only come to light since he has decorated and changed the colour from white to this grey colour. The conservatory is over twenty years old and this is a cavity wall.   I have tested the wall with a damp meter and it’s telling me that there is only a 1% difference between the light and dark areas. 

 

From visual inspection, the DPC is intact and well above ground level.

 

Any help or advice would be greatly appreciated.

[SteamyTea]

Probably a chemical reaction between the higher alkaline mortar and the paint.

They should be able to find a sealer that cures this, but it will be trial and error (or a chemistry set).

 

[Tadders]

Thank you SteamyTea. 

I really appreciate your advice.   I had not thought of that being a possible cause. 

However, I have been into visit my neighbour and the two smaller walls under the windows of the conservatory that were constructed at the same time are not showing any evidence of these darker areas.  The same mortar would have been used for all the walls.

Thank you once again.