Expert Advice
Frank DiStefano

How to Minimize Tannin Leaching

Your timber decking or cladding can leak onto the surrounding surfaces because of timber leeching or tannin bleeding.

Tannins are one of the many natural extractives present in timber. They’re water-soluble and are released from the timber upon getting wet. The leaking reacts and stains surfaces like concrete, render and pavers making the timber surround unsightly.

Example of tannin leaching

All hardwoods contain tannins, however, some species are more prone to leaching than others. For instance, timber like Merbau and Blackbutt are more susceptible to bleeding tannin than others like Ironbark, Spotted Gum and Cumaru. Mostly tannin stains are brownish in colour, but Merbau produces a red-brown stain that looks even worse. Exposed timber structures, especially decks surrounded by concrete, pavers, or renders are affected by tannin leaching as they’re likely to get stained. If you’re deck is surrounded by any of the above, then you should consider choosing low tannin leaching species to avoid staining. 

Although tannin tends to leach from timber only for a short time and is generally not a long-term concern, it should not be underestimated. High extractive timber, though painted can also show signs of leaching and staining. However, the good news is there are multiple ways to minimize tannin leaching.

Use Tannin Blocking Primer or Tannin Removers

For painted timber, high-quality tannin blocking primers are available in the market to reduce timber leeching. These primers should be used on the timber before applying the topcoat. Fully coated decking boards do not allow moisture to penetrate, thereby limiting tannin release on surrounding surfaces. Coating timber not only help lock extractives but also keeps it in a pristine condition for years to come. 

For oiled timber, Tannin remover and deck cleaning products can be used repeatedly to wash out tannin stains. Once the timber is washed with the right cleaning agents and finished with a protective coat, it’ll automatically reduce leaching.

Pre-Coat the Timber to Maintain the Deck

It is very important to maintain the coating because it can eventually break down from UV and wear traffic which lets the moisture back in, allowing the tannins to bleed out. Receiving timber that is factory pre-coated with Intergrain Universal Oil from Bowens on all sides prior to installing can help minimize the leaching process.

Alternatively, you can leave the high tannin timber decks raw over a winter season to leach out. You can do this if your deck is surrounded by a garden or lawn where leaching can run into the soil without causing any noticeable issues. Once the tannin leaching is reduced you can clean the deck and apply a finish coat to restrict further bleeding.

It is ideal to choose kiln-dried timber for outdoor construction if you’re concerned about leaching. Kiln-dried timber ensures stability and minimizes tannin bleeding compared to unseasoned timber. Ultimately prevention is better than cure, so the right thing to do to minimize leaching is to pre-coat and maintain coatings, apply tannin removers or blockers, or select low tannin leaching species in certain designs.

For more information on decking and cladding maintenance, get in touch with Bowens customer service team today. 

Click here for more information on Bowens Pre-coated Timber service. 

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