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#24 Cleaning Barn Timbers

I'm in the process of converting a 150 year old barn into a residence in Cape Cod . The interior beams are Douglas Fir and have significant stains from dirt and other agriculture related materials. I was wondering if oxalic acid would be the best route to pursue, or if the toxicity of it is such that an interior application would be unwise. The beams themselves will remain exposed in living spaces so aesthetics are as important as safety.

Ha! I like your euphemistic phrase, "other agriculture related materials." We can speak plainly here, and if we're dealing with horse manure, we can say “horse manure.” One of the most important aspects of cleaning is to understand what the dirt is made up of. So, if we have manure, then we not only want a good looking surface, but one that is disinfected and clean enough to be healthy to live with.

If you want to preserve the character of the beam surfaces always test out least aggressive cleaning methods first. For example, simply wash the surface with warm water and a coarse rag such as burlap. If that's not effective, try using a soft bristled brush; not so good? Use a stiff bristle scrub brush. Still could be better? Add just a capful of mild detergent (ordinary dish washing detergent like you use at the kitchen sink) and rinse with plain water. OK, there are still deposits of grunge and grime? Use a low-powered detergent, such as Simple Green. Not good enough? Then try a high-powered detergent such as Tri-Sodium Phosphate, but first mix it at half the recommended concentration. See? Least to most aggressive, just stop in the testing process when you get the results you are after. Big Benefit: you'll do the least amount of work and get the biggest bang for your buck.

One reason to use a scrub brush rather than a high-pressure power washer is that you want to contain the dirt so you don't spread the contamination all around with a powerful blast of water. In any case, wear protective clothing, respirator, goggles, face shield, etc., when doing this sort of cleaning; and follow effective personal hygiene and cleanup practices after every work session. On large cleaning jobs you can step up production by applying cleaning solutions and water with an ordinary low-pressure garden sprayer or garden hose and nozzle. Also, the 3M “DoodleBug” scrubbing device on an extension pole will increase production. Often the high-pressure power washers cause damage to the wood surface. It is possible to operate a power washer without causing damage, but I have seldom seen one used with the "artistic nuance" required.

OK, after cleaning, if you've still got stains and uneven appearance you can try more aggressive chemical treatments, which will work better on the clean surfaces. I would start by testing ordinary household bleach in a 1-part bleach to 20-part water solution, stepping up the concentration gradually to not more than 1-part bleach to 5-parts water. This will kill some types of unhealthy biological organisms and do away with some types of surface stains, evening out the appearance. No good enough? Then try a two-part oxygen bleach system.

Then you might need to try oxalic acid. Oxalic acid must be treated with caution and respect when it is used, but interior use and surfaces treated with it are relatively safe after treatment. When using any chemicals (even relatively safe and familiar ones such as dish detergent) always follow all of the manufacturer's instructions, and the manufacturer's Materials Safety Data Sheet for that product.

Keep in mind that this is a barn, and you are seeking a balance between the modern sense of "brand new" surfaces that are acceptable in a living space and what actually is a rough service area intended for animals. It is probably that rough character that initially caught your interest, but if you clean it ALL away you may loose the character that seemed attractive to you in the first place.


John Leeke is a preservation consultant who helps homeowners, contractors and architects understand and maintain their historic buildings. You can contact him at 26 Higgins St., Portland, Maine, 04103; or by E-mail:; or log onto his website at:

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