I can't see that it's possible to "trap" water between 2 coats of
something permeable. But if water pours in at a specific point when it
rains and then has to dry out gradually by evaporation, the wall may
well give the impression of being permanently soaked.

I was using the word 'trap' loosely. The wall is enclosed by 4:1
cement render. In this part of the world just about everything is a 4:1
mix. I didn't put it there, nor would I have, but I've learned from
experience that the locals have their own way of doing things, based on
their experience.

As I said previously, we've had little rain for a long time but, when
it happens, the wall gets no wetter.

Tony.