what is the best wheather sealant for an outside wall

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Hi all
I need to know what is the best weather sealant for penetrating damp to put on an outside wall. My builder recommended dulux weathershield with johnson weatherproof on top.

But there are so many 'amazing' products on the market and I want to find the best one.

Any recommandations?
 
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thanks, John, I looked this up and it really looks interesting. but there are other sealants that make similar claims and are cheaper.

Do you have experience with this? What are your reasons?
 
Yes, I've specified it a lot. Mainly for historical buildings because it has exceptional breathability and is invisible when applied correctly. It is a bit more money but it is a once only treatment which can't be removed so I wouldn't take any chances.
 
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thanks, John, your advice is much appreciated.

We are having a lot of trouble with a cavity wall that is facing the weather.

The previous owners have applied a thin coating of some weird material that has not really bonded with the cement render underneath. So, we want to sand-blast that off (it easily flakes off) and then apply the sealant on top of the existing render.

How many coats do you recommend?
 
Always follow the product manufacturer's specification for preparation and application. K501 is usually two coats. It can be by brush but I prefer spray.
 
do you need an undercoat? what do you recommend for that?
 
Reading through your posts, you have a cavity wall (facing the weather = South westerly?) with an external render, with a coating that is flaking off. Internally, you imply that have damp which you believe is penetrating.

If you want to recoat the decoration of the render, removing the flaking coat may be a good idea, but it will do very little for the damp inside.

How is the moisture bridging the cavity between the external and lnternal leaves of the wall?

If the cement render is inappropriate or breaking down, rather than protecting the wall it may be causing problems. Older brick walls were built to breathe, so that any moisture would evaporate uniformly, and lime mortar was similar in behaviour. Pointing these walls with hard, impermeable, cement mortar accelerates mortar course decay and spalling of softer bricks during cold weather. This is because the water expands as it approaches freezing point, and more as it freezes, and can take off a layer of brick, mortar and pointing when thawed.

Similarly, cement mortar over older brickwork can prevent the bricks from drying, leading to hidden damage to the wall when the weather turns cold. This may lead to blown patches of render, cracks, more water ingress and finally the render coat will shear off.

Modern brickwork with cement mortar and a cement render is a much more uniform and impervious combination.

So what is the age of the house, orientation of wall, composition of bricks and mortar, cavity size, whether any cavity wall insulation has been inserted, and what type and when, state of wall ties, any cavity inspection to see if there are mortar bridges.

It would be helpful to describe the damp, location in room - what type of room, etc perhaps with a couple of photographs.

If the damp is penetrating, a masonry drill hole into the damp area will produce muddy paste, but if the damp is surface only, the drilling will produce dry dust.
 
Flyboytim
thank you so much for your reply.

There have been so many problems with that wall that I almost can't count.

It's a cavity wall of unknown age facing south-west in a house of numerous botched-up jobs.
We will make some investigations of the kinds you have recommended and we have also a man coming to install cavity trays above the doors and windows.

Three builders who have seen the wall recommended to leave the cement render saying it looks quite good.

I understand the argument about cement render being a problem but hacking off the render means that the bricks then need to be re-pointed making it a huge and expensive job.
 

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