You’ll want to be aware of this when painting or staining wood. Let’s consider you’ve selected your wood surface and your stain and you’re looking forward to having this beautiful wooden whatever. But something happens … blotchy brownish or tan stains start to appear on the surface you’ve just painted over! If you think that all it needs is another coat of paint or two, think again. It doesn’t matter. Those stains are not going anywhere. You have just experienced tanning bleed, and it is a fairly common occurrence. So – – what can you do about it? Can it be prevented? Keep reading and we’ll share some tips of the trade.
What are Tannins and Why does Tannin Bleed Happen?
Tannins are a kind of complex chemical which can be found in many different plants, including trees. More specifically, in the bark and wood of trees. Tannins inside of tree bark help to protect the tree from fungal and bacterial attacks. Tannins can be used to treat several ailments like tonsillitis, skin eruptions, and can be used as antidotes for glycosidic and metallic poisons.
Though there are undoubtedly beneficial uses for tannins, the substance can be a big headache for painters due to the tannin bleed. Tannin bleeding can happen with every type of wood, but redwood, mahogany, and cedar are exceptionally prone to staining in this manner. As for what causes it, tannins are either water or solvent-extractive (they absorb water or metals), with most kinds of tannins being the former.
As it turns out, latex-based paints aren’t very tannin-stain resistant. This means that applying latex paint directly to wood of this type can result in tannin bleed since it has a high water content. That means, the wood soaks up the water in the latex paint and this, helps draw out the tannins.
Tannin bleed can also occur if there’s a lot of moisture inside of the wood’s surface. Typically, in these instances, the staining will occur a while after the paint job has been completed. The water will become inundated with tannins, which then travels from the surface of the wood to the surface of the paint.
How Can I Prevent Tanning Bleed?
Fortunately, tannin staining can be prevented and even corrected once it appears. One of the best ways to avoid bleed-through is to apply either an oil, or alkyd-based primer, or shellac. That being said, even with an oil/alkyd primer, tannin bleed can still occur.
Shellac seems like the better preventative option as many painters have found stains much less likely to appear once they’ve painted over it. Regardless of which of these sealing methods you use, it’s best to err on the side of caution and apply two coats.
If you’ve already got stains, then you’re going to need to find where the source of moisture is, fix it, and let the wood dry for at least 48 hours. If the paint is peeling off the wood’s surface, then remove it with a scraper or wire brush.
The stains themselves can be taken care of with oxalic acid (it breaks the stains down into something that can be easily rinsed away) and then wash them away with a pressure washer – again, be sure to let the wood dry for approximately two days. After you’ve done all of that, you can prime the wood with two coats of oil/alkyd or shellac and repaint it.
We understand, tannin bleed can be terribly frustrating, even more so for people who have started a painting project and may not know what’s going on. Tannin stains are a surface issue, however, not a problem within the wood itself.
Hopefully, now that we’ve told you what tannin staining is and how to prevent or resolve it, painting has become less of a pain and more something you enjoy.
