Oxalic acid is a gentle stain remover that eats the stain but leaves the base surfaces such as wood, intact. Oxalic acid is also used to remove rusts that are commonly found on plumbing pipes and kitchen countertops. You can also use Oxalic acid to remove stains from fabrics such as linen and cotton Nutritionists refer to oxalic acid as an anti-nutrient because it interferes with the essential minerals, iron, magnesium and especially calcium. Earlier this century spinach was advocated as a rich source of iron, and indeed it has higher levels of this metal than most vegetables.