This article acknowledges the possibility of painters’ shortcomings but deals mainly with the mysteries behind the above.

First, the most common causes of PAINT BLISTERS, BUBBLES, CRACKS AND FLAKING: 

  1. lack of preparation
  2. Moisture and salts below the surface
  3. Excessive heat 
  4. A combination of 2&3
  5. A combination of 1,2&3

Preparation is simply what it says on the tin. Some tins are more instructive than others but basically ensure the surface is scraped back until stable, is clean, holes and cracks are sorted and powdery surfaces handled with a coat of stabilising solution. Then open the paint and have fun.



Sometimes even with thorough preparation one or more of the above can occur. Why?

Well ok, let’s take wood: It could be the wood was resinous after stripping down and needed a couple of coats of knotting before painting. 

Knotting is that clear smelly stuff you might see a decorator applying to the  knots in new timber.  Without this application you will get (after some months) yellow stains coming through the nice white paint on the doorframe.

But some external doors are just heart-breakers. You strip them down,  taking all precautions and still they bubble in the June sunlight.

Well June is a lot of photons being converted to heat on a black painted door and often after that brief threat of summer the bubbles disappear. 

Yes, they go away as mysteriously as they arrive. 

So what to do? 

Strip off the bubbles, destroy an otherwise beautiful paint job, and try again?

 Or leave good enough alone and wait till autumn when the bubbles will go away?

I have such a door on my own house; I know how it was painted and thus familiar with its history, I leave it alone. The bubbles come and go with sunshine and the surface never cracks. And they’re at the bottom of the door so no one but me and the maybe the postman ever sees them.

Moving onto painted render, stucco and stonework, similar problems can arise, even after good preparation. 

Sometimes the moisture and salts in the render are not evident until the coat of paint is applied and exposes these things

Then you simply scrape it back and try again. Mostly it works. If it doesn’t then probably the chemical structure of the render has been broken down by water penetration and the render has become hydroscopic. What to do about such a big word? Hydroscopic means it can absorb water. Which means that section of render needs replacing.

That’s about it, but just now looking over what I’ve written, I have to say this:

If you strip down an old external door, study it carefully for any signs of resin, newly exuding after release from 100 years paint coverage. 

There you may find the source of that mysterious bubble.

Good luck