Black marks on new hardwood suggest that the problem is 'iron staining', ie. metal particles falling on the deck and reacting with tannin in the wood when it gets wet. The recommended treatment is oxalic acid, or a proprietary timber cleaning product that contains oxalic acid. The US Department of Agriculture's Forest Service has a helpful data sheet on the subject. It can be dowloaded via this link https://www.srs.fs.usda.gov/pubs/48449. Treating the timber with an oxalic acid solution is likely to be easier and more effective than the builder trying to sand the marks out. You may be able to stop further staining if you can find the source of the metal contamination and eliminate it. It could arise from cutting metal nearby, for example working on guttering or other metal items. Your builder can find a product with an oxalic acid base by checking the labels of deck cleaning products, or reviewing their Material Safety Data Sheets.Iron stain
Answered on 15-04-2021
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