There are several reasons stains, marks & discolouration appear on your deck. We'll go over a few of the common problems and see what can be done about them. 1: Tannin Stains What is Tannin? Tannin is an natural organic preservative produced in many plants & trees to help deter predators, with some species having a higher tannin content then others. And yep, it's the same stuff that stains your teacups and gives your a red wine a dry mouth feel. Tannin stains often become vis