Paint application errors: Paint vs Painter
Things can go wrong between a high-tech, carefully manufactured paint vs the painter that is applying it. Most coating failures are due to application errors rather than problems with the paint.
I’m not a chemist, or even particularly well read around paint technology. But I have picked up a few things that are important to my job.
A common misconception is, that employing low-skilled painting labour can be compensated for by buying “better” paint. In most cases the opposite is true. A good painter will make cheap paint look good and last a long time.
So, what is this magical knowledge a skilled painter can bring to bear on your rough old woodwork? I thought I would talk about just a couple of examples from the deep world of paint!
First, an easy one.
Big cutters saw, plane and mould the wood to the final dimensions of the cabinet or door or window. The joiners don’t always sand the joinery very thoroughly (if at all). This results in sharp corners at the edges which, as I will show you, are a hotspot for paint failure.
Imagine looking through a microscope at a cross-section through one corner of a piece of new joinery. And imagine that an inexperienced painter has painted one coat of a fetching shade of orange paint onto the joinery.
The surface tension of wet paint makes it flow away from a corner leaving a thin layer of paint. The thin layer of paint is vulnerable to damage. The painter could accidentally sand it off before the next coat of paint.
If, on the other hand, the painter (or Joiner) had sanded the joinery to slightly round off the corners before painting, the paint will cling to the corner more evenly, leaving a more durable thickness of paint.
This one’s a bit more complicated – stick with it!
The Manufacturers Data Sheet (or back of the tin) for a product usually gives advice on the necessary drying conditions for using the paint. This will include a minimum and maximum temperature, and sometimes maximum Relative Humidity levels.
To illustrate one potential for paint application errors. Let us say your painter is painting an emulsion type masonry paint outside late in the year. The weather is a bit chilly, and he needs to finish the job today. So, even though it’s getting a bit cold, he carries on to a finish. It’s not going to rain tonight. Our painter is confident it will dry, finally, sometime in the morning when the sun gets up.
To understand the potential problem, first a bit on how emulsions dry.
How paint emulsions dry (simplified)
An emulsion paint is mainly comprised of what I am going to call blobs of polymer suspended in water. Imagine the blobs of polymer as like big balls of wool. (I will explain the red dots later).
Under good drying conditions the solvent (water) evaporates and the polymer blobs are forced together. The strings in those balls of “wool” get tangled together.
When the water has evaporated, the polymer blobs join together (or coalesce ) forming the paint film.
The difficulty for paint chemists is the polymer blobs don’t really coalesce with each other very easily. Particularly under cool temperatures.
Now, this problem can be overcome by formulating a polymer that actually will coalesce at low temperatures. But this would result in a soft and sticky paint film, even when cured.
(Now the red dots!) To solve this problem, the chemists add a small amount of Co-Solvent to the polymer. The co-solvent is a volatile chemical mixed into the polymer to make it less viscous. So now, when the polymer blobs are coalescing (at the cool temperature) they will fuse together more easily. This results is a polymer that will form a good film at cool temperatures.
Why this is important
You will remember our painter is painting late in the day, and is hoping that the paint will dry some time next morning. All the time the wet paint film is sitting there during the night and the water is staying put, the co-solvent -being volatile – could still be evaporating. By the time the sun gets up, all the co-solvent could have gone before it’s warm enough for the water to start drying. The polymer blobs will not coalesce very well. This results in tiny air gaps and a non-uniform film that make the paint weak and porous, and could ultimately result in premature failure.
These are only two examples of the many ways paint can fail due application errors by the painter. So….. always read the back of the tin, and if it it states limitations in application conditions, it does so for a reason!
Published by: Colin Taylor on: December 23rd 2020