Because many ready-made products contain mainly water and little oxalic acid, we choose to supply only the active oxalic acid itself. Not only does this save on transport costs because no water needs to be moved, it also makes it easy to make the strongest solution possible.

Oxalic acid comes in crystal form and is dissolved in water after which it is also known as design water or weather spot cleaner. You work with oxalic acid as follows:

• Dissolve about 100 grams of oxalic acid crystals in a litre of water. Warm water works fastest; for cold water, leave the solution for a day and shake occasionally. If not all crystals dissolve, this is not a problem. You have then obtained the maximally saturated solution, or the strongest solution you can make. As long as there are undissolved crystals in the jar, you have the maximum strength of the solution and simply add new water if the level drops during use. Over time, when all the crystals have dissolved, the concentration becomes less and it is time to make a fresh pot.
• When using larger quantities, dissolve 1 kg of oxalic acid in a 10-litre bucket of, preferably warm, water. At that ratio, you have about the strongest possible concentrate and can get started almost immediately.
• Apply the solution to the discolouration with a flat brush and allow it to soak in.
• In case of serious soiling or greying, brush the surface well with a clean Coconut brush.
• If necessary, repeat the treatment until the discolouration disappears.
• If the discolouration has disappeared, a light spot may have replaced it.
• Now treat the entire surface (not just the discolouration) with the solution and let it soak in again.
• Finally, wash thoroughly with clean water to neutralise the acid and remove any crystals remaining on the surface.
• Allow to dry and further treat the surface with a product of your choice.