Exterior Waterproof

In my opinion, trying to seal in the rising damp isn't going to help, as it will in time force off whatever coating you put on.

What I have found to be helpful is to tile the wall, using flexible cola - not ordinary cola. The damp might then rise to above the tiles, depending on how bad the problem is. On one wall I started with a single row of tiles, but i'm now up to six rows. 

The only real solution is to stop the damp rising by drilling lots of holes as low as you can so the moisture can escape.

Thanks Jim the problem is not rising damp it's the render is of poor mixing quality And the rain is penetrating and reaching internal brick and subsequently the plaster. Ideally the rendering should be removed and proper one applied, I won’t hold my breath for the builder to do this.

Apologies, I'm so used to rising damp I thought that was the problem, aggravated by rain. I suggest you ask at a good paint shop. I don't know your area, but there's a Brochacolor in Torrevieja. Getting it re-rendered with an exact colour match could be a problem. 

My inclination would be to give the render a good soaking with liquid silicone or similar. That would seal it and prevent the ingress of water, but would still leave it breathable. 

As you suggest, you could also seal the surface completely with an impermeablizante, but appearance could be a problem. There is a red water proofing paint I used to use on my flat roof that might not look too bad. It really is a question for the experts, and Brochacolor have always been very helpful, so perhaps a trip to ask them.