I have a bloom of colour that looks like lichen,forming on a wall I have just painted. It started quit small and has grown over a couple of hours. What could cause this?
Blooms of colour like this that have the look of lichen growing on rocks, tend to be caused by painting in cold weather where both the air temperature and the substrate temperature are low enough to retard the drying time of the paint to the point that salt crystals are able to form on the surface of the paint. The growth of these "lichen" patches of salt crystals is quite fast, with each bloom growing from a pin head size to the size of your hand in less than half a day.
If you are unlucky enough to experience these crystal growths don't fret to much. If you can warm both the air temperature and the substrate temperature up (this may take several hours of heating the room to get the walls warm), wipe the affected areas with a damp cloth, all you need to do is recoat the affected walls.
Most Acrylic wall paints have a minimum painting temperature of around 13 degrees. This ensures that both the water in the film evaporates and the acrylic emulsion coalesces to form a hard film.