thick_mike":3ob95jh9 said:
Jacob":3ob95jh9 said:
Beau":3ob95jh9 said:
Used to be an International paint called Ranch Paint. It was breathable and did my workshop doors for 15 years. Came to recoat them with the supposed replacement and it has trapped water in and caused the doors to rot.
Can see the value in finding a quality breathable paint.
I'm told that all paint is 'breathable' and 'microporous', unless you go to extremes with bitumen, gold leaf etc. The words are bandied about by sales people and have as much/little meaning as 'new, improved'
You are correct, any finish you use will allow water through to some extent. I used to formulate car paint for OEM and refinish and we had to test water soak and humidity exposure. Despite them being very sophisticated 2 pack coatings baked at up to 130C they still absorbed moisture. Any coating on wood will allow some water to pass through.
Yes seems to be case just some are more microporous than others. Thought this was interesting, it's from ICI so bias but still worth a read
The term "microporous" is often applied to specialist paints and stains and describes a coating that acts as a barrier to liquid water, but allows water vapour to pass through. The implication being that there is something special about the coating that keeps rainwater out of the substrate, but lets moisture that has found its way into the substrate escape as a vapour. Although almost all coatings will act as a barrier to liquid water, it is not correct to imply that coatings can be made so that moisture vapour can only travel through them in one direction. Moisture vapour will be able to pass through a coating equally well in either direction: from places where there is more moisture, to places where there is less moisture. The rate at which moisture passes through a coating is controlled by the permeability of the coating, the moisture content gradient, the film thickness, and the temperature.
ICI Paints has avoided using the term "microporous" to describe its products, regarding it as a somewhat misleading sales gimmick, because nearly all paints and stains offer some protection against rainwater, and all paints and stains will to a greater or lesser extent allow moisture vapour to pass through them in either direction. Instead scientists at ICI prefer to speak in terms of appropriate levels of "moisture vapour permeability": for example on a timber substrate, too little can result in paint blistering or worse still wood rot can begin to occur, too much allows moisture to pass into and out of the timber too easily, leading to splitting and cracking of the wood. So it is not just a question of the more the better. Moisture vapour permeability is one of a number of important attributes that must be considered when formulating an exterior coating - others include: adhesion, cohesion, flexibility, mould resistance and photostability.
Microporosity is therefore not some miracle technology, it is an invented word that has been used to describe paints and stains that are usually quite high in moisture vapour permeability - sometimes excessively so. Often such wonder products are single paints said to do the whole job in one go. We believe that this approach is grossly over simplistic, and prefer to take a much more systematic approach to achieving lasting durability with coatings for exterior wood, particularly where redecoration is concerned. For example, each part of the Weathershield Exterior Gloss System is formulated to do a specific job, and has a carefully balanced set of properties, including a suitable level of moisture vapour permeability, working together they help to keep the moisture content of the wood below levels at which timber decay can become a problem.
The Basics
Water Vapour Permeability
"Microporous" - "Breathing" - "Permeable"
All paints can claim to be microporous.
What is more important than microporosity is the following properties:
Mechanical Properties - flexibility
Adhesion
Biological resistance
Chemical resistance
Resistance to degradation by sunlight.
Early shower/water resistance
Resistance to soiling/dirt pick-up.
Compatability with previous coatings and where appropriate glazing seals.
