Tannin Wood Discoloration - Causes & Solution
Over time, wood stain discoloration may appear in woods containing tannin, such as cedar, Douglas fir and mahogany. This is because the natural chemicals in the wood slowly migrate to the surface. This wood discoloration is a source of frustration for people who work with lumber, because it seems to happen completely by itself. It appears as a granular, brownish stain that is most visible on light woods.
Tannin is a natural chemical, and is part of many trees before they are harvested for lumber; occasionally, tannin discoloration is visible in living trees. Moisture tends to soak into wood over time, which gradually pulls the tannins out. Any source of moisture can cause this, from rain to fog and mist; if the coatings on the wood do not provide a watertight seal, tannin leaching is an eventual certainty. Sometimes, puzzling tannin stains can appear on woods that have little or no tannin in them. This tannin spreads from other woods, such as roof shingles directly above the tannin-free piece.
The principal way to stop tannin from migrating to the surface is to use a primer specifically designed to prevent it. Many modern primers mention tannin in their documentation. Primers that block moisture also have the added benefit of preventing water-related damage that may warp or bend wood. It is important that the target wood piece is fully coated; if there are small gaps in the protected surface, significant moisture eventually gets through. If tannin has already shown up, since it is on the surface, cleaning it is straightforward with products made by home-improvement companies like Sherwin Williams.
Tannin discoloration is often surprising to people who did not know that discoloration could spontaneously appear. However, with the right products, it is easily prevented from showing up in properly-treated lumber.
Primers and Finishes
Priming is critical. Stain will bleed through multiple top coats without proper priming.
Select a top quality exterior paint in the color and sheen of your choice.
Primer: 255 Stain Lock II Acrylic Primer
Finish Coats: 1240 Acry-Shield Exterior Flat Finish
1245 Acry-Velvet Ext. Low Sheen Finish
1250 Acry-Lustre Ext. Semi-Gloss Finish
1235 Seasons Ext. 100% Acrylic Flat
1236 Seasons Ext. 100% Acrylic Low Sheen