The following information is from Forest Products Laboratory’s Wood Handbook, Wood as an Engineering Material.
Iron stains occur from rusting of fasteners or by the reaction of iron with tannins in wood. The appearance is different for each of these reactions.
In wood species that lack tannins, iron merely rusts, giving a brown stain to the wood surrounding the fastener. The iron also causes slight degradation of the wood near it (often referred to as “wood sickness”). This discoloration develops over many months or years of exposure.
In wood species that have tannins, a chemical reaction takes place between the iron and the tannins. Tannins are just one of the many chemicals (extractives) in wood. Species such as the cedars, the oaks, and redwood are rich in tannins. Iron reacts immediately with the tannins to give a blue-black discoloration.
Steel fasteners are the most common source of iron, but traces of iron left from cleaning wood with steel wool or wire brushes cause iron stain. Poor quality galvanized nails corrode easily and, like uncoated steel nails, usually cause unsightly staining of the wood.
Using the wrong fastener can be costly—it may become necessary to replace all the siding. Therefore, your best bet is to use corrosion-resistant fasteners, such as stainless steel, rather than risk iron stain, particularly when using natural finishes on wood containing high amounts of tannin (such as western redcedar, redwood, and oak).
If using galvanized fasteners, they must be hot-dipped galvanized fasteners meeting ASTM A 153/A specification. Other galvanized fasteners fail. Unfortunately, contractors and their employees may have difficulty recognizing the difference among galvanized fasteners.
Can iron stain be fixed?
If iron stain is a serious problem on a painted surface, countersink the fastener, caulk, spot prime, and top-coat. This costly and time-consuming process is only possible with opaque finishes. Little can be done to give a permanent fix to iron stains on wood having a natural finish.
Removing fasteners, cleaning the affected areas with oxalic acid solution, and replacing the fasteners may not give a permanent fix because residual iron left behind continues to cause staining. Removing the fasteners often splits the siding.
Iron stain occurring beneath a finish is extremely difficult to fix. The coating must be removed before the iron stain can be removed. Oxalic acid will remove the blue–black discoloration. Apply a saturated solution (0.5 kilogram of oxalic acid per 4 liters of hot water) to the stained surface. Many commercial brighteners contain oxalic acid, and these are usually effective for removing iron stains.
After removing the stain, wash the surface thoroughly with warm water to remove the oxalic acid. If even minute traces of iron remain, the discoloration will recur.
To find out more about iron stain, and what can be done about it, please see this FinishLine.