What primer sealer undercoat for powdery porous concrete render?

[Lastbrick]

We are about to repaint (new colour) the render walls of our house and fence. These walls face both south and west so get a lot of sun. 
The surface is powdery and retains water, but it is not flaking / friable. Some walls were oriiginally painted 15 years ago, others had 1 coat of Wattyl Solarguard applied 6 years ago. We have a lot of area to cover and are trying to keep the cost down while still getting a reliable result. 

We're open to other suggestions as to best products but the 2 under current consideration are:
British Paints Prep 4-in-1 and Dulux 1 Step (both primer, sealer & undercoat).
The Dulux product is only about $30 more than the British Paints when purchasing 15L. 
Also we're not sure what paint brand to use yet, so it may not match the primer brand. Not sure if that's a consideration? 
Would appreciate your advice. 

 

[EricL]

Hello @Lastbrick 

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thank you for sharing your question about your concrete walls.

If the walls are powdery and retain water, is the water coming from the other side of the wall? What is on the other side of the wall that could be causing water from passing through? Please let me know if I have misunderstood the situation. If it is just surface water then you'll just need to wait for the wall to dry off. But if the moisture is coming from within the wall and coming to the surface, it is either a leak in the plumbing or water bridging or passing through the wall from an unknown source. 

This leak must be addressed first before you seal the surface with paint. Otherwise painting it now will result in the paint peeling off due to hydrostatic pressure from the wall. If you suspect that it is a leak I suggest engaging the services of a leak detection specialist. They'll be able to track down the source of the water passing in your wall.

However, to answer your question, I advise looking at using Dulux 4L PRECISION White Sealer Binder - 4L or Zinsser Peel Stop Binding Primer Clear 1L, both are designed to seal dusty, chalky surfaces. The major difference between the two is that Zinsser is water-based which makes it easier to clean. The Dulux Precision is oil-based which takes longer to dry and recoat and you need to use turps to clean your painting equipment.

In regards to the top coat, I suggest matching it with the primer based used, water-based or oil-based top coat. I recommend looking at the Dulux Wash and Wear or Taubmans Easy Coat.

If you need further assistance, please let us know.

Eric