Tannins are bitter vegetable compounds that add piquancy to your glass of wine or cup of tea. They are also present in some kinds of wood, and that's when they begin to have something to do with painting and decorating.
Specifically, tannins create stains when they are activated by moisture. That means that your nice freshly painted or stained wood can have great blotches that come back no matter how often you repaint them. This is called "tannin bleed."
What causes tannin bleed?
- Using a water-based undercoat when the surface isn't suited to it, including when you're painting over oil-based paints, can lead to this problem. Water activates the tannins and causes them to migrate to the surface of the paint, creating stains some time after painting.
- Moisture from other sources, including indoor damp, leaks that send moisture into the walls, or high humidity surroundings can also cause tannin bleed.
- Uncured or poorly primed wood is a common source of this problem.
Thoroughly cleaning and drying the surface before painting and then using a stain-killing primer can reduce the likelihood of tannin bleed in humid areas. If you have reason to believe that tannin bleed could be an issue for your painting job, be sure to let your tradesman know ahead of time.