Cottage Conundrum: FPL Fields Cleaning Question

All of the research and expertise of the Forest Products Laboratory (FPL) is in vain if we can not use it to better the American people. Our lab receives many questions for our researchers, and recently, we received a question regarding T1-11 cedar siding.

Dear FPL,

I have a problem with the T1-11 Cedar Siding (plywood) that was installed on my cottage back in 1986. There is a black fungus on all sides of the cottage, even where the front is exposed to the direct sunlight most of the day. I would like to have a vinyl siding installed over the T1-11, but am not sure if I should have that done without washing the siding first. I have washed the siding in the past and the fungus has returned! I would appreciate any advice you can provide to me.

Cottage Conundrum

Luckily, Mark Knaebe, a Natural Resources Specialist with the Forest Products Marketing Unit, had the answer!

Dear Cottage Conundrum,

The most important question here is what do you really have?  If you used what is commonly called a “wood bleach” which is oxalic acid, and it brightened up, then you have iron stain. Iron stain is not a fungus.  

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Although it leaves wood black, iron stain is not a fungus. The above example was caused by corroded fasteners.

If you used a chlorine or oxygen bleach and it brightened it up, then you have mildew, which is a form of fungus, and it may not be a huge problem except it that it can keep wood wetter for longer periods which can promote decay fungus (rot) in very wet areas.

I would not give up on the T1-11 just yet.

If you are set on putting vinyl over it, and it is iron stain, you can ignore the black. If you have mildew, you should find out why first, and if bleach cleans it up, you could just bleach and put up the new siding paying special attention to proper flashing. Traditional penetrating coatings contained oils which are food for mildew so as soon as sunlight destroys the mildewcide, mildew can quickly return, so that might be what you’re experiencing.

The bottom line is find out what it is first, and take the next steps from there. Thanks for your question!

Mark

 

Choose the Right Fasteners to Avoid Unsightly Iron Stains

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.

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Iron stain on newly installed wood siding. Poor quality galvanized nails corrode easily and, like uncoated steel nails, usually cause unsightly staining of the wood.

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.

For more information, please see chapter 16 of FPL’s Wood Handbook, Wood as an Engineering Material.

To find out more about iron stain, and what can be done about it, please see this FinishLine.