Regardless of what caused the chemical stain on your deck, start by using the gentlest removal process feasible work your way up to Boundary procedures. This way you may have the ability to avoid reapplying a coat of sealant over the area.
Start Gentle, Get Tougher
Clean the stain with a scrub brush and a deck cleaner that contains oxalic acid. Make your own cleaning solution by mixing 12 ounces of oxalic acid crystals in 1 gallon of water. If the oxalic acid doesn’t remove the stain, rinse the place thoroughly and attempt a mixture of 6 ounces lye, or sodium hydroxide, in 1 gallon of cold water. Lye can lift the finish, so the deck will need to be resealed when you’re finished. Ensure to wear protective gloves and masks when using deck-cleaning options and rinse the deck thoroughly afterward. If lye doesn’t remove the stain, it can be sanded off with fine-grit sandpaper. Work in the direction of the grain and sand only until the stain is gone. You will need to refinish or seal the portion of the deck following the stain is removed.