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 visible after rain as the tannin is naturally drawn to the surface as in the above photo.
Combating tannin starts with a weathering process.
Leave your deck bare for around 6 to 8 weeks before finishing. This allows the resin, tannin, wax and natural pigments (all called extractives) to bleed out.
Hosing the deck down during this process can help remove the tannins, but not if there is very hot weather or a heatwave which can lead to splitting or cupping.
After the weathering process (and before finishing), use Napisan (or a deck cleaner containing Sodium Percarbonate) mixed in a bucket of hot water - about 2 cups of Napisan to half a bucket of water. Commercial deck cleaners are a good choice, but Napisan will do the job, is readily available and inexpensive.
Apply the mixture to the deck with a hard bristle brush or broom, leaving it for around 15 minutes before washing it off well with the hose. This method may require a few washes & scrubs to get most of the tannin out of the boards.
Allow to dry for a couple of days before finishing.
This is standard good practice every 6-12 months as discussed in our last blog about deck maintenance to keep your deck in great shape.
2: Black Marks
There's a few reasons these might appear:
If welding or grinding occurred next to the timber, tiny bits of iron filings can land on the timber and react with the tannins, causing the surrounding area to go black.
If metal objects were left sitting on the deck for a period - such as nails or tools.
Areas around the heads of nails or screws (as pictured above).
Use a commercial deck cleaner containing oxalic acid - commonly sold under the name of 'grey deck cleaners'. Check the directions for the correct ratio to water as most need diluting.
Apply the mixture to the deck with a hard bristle brush or broom, leaving it for around 15 minutes before washing it off with a hose.
It may require 2-3 washes and a fair bit of elbow grease do get the timber looking like
3: Mould, Mildew & Algae
Mould or Green Algae is a very common problem on decking, especially if your deck is shaded, and isn’t something to be overly worried about as far as the condition of the timber - but it can make it slippery to walk on, and leave a drab grayish surface.
Sweep the deck thoroughly with a stiff brush, making sure you get right into the corners where mold and algae have a tendency to build up.
Look for a specific algae/mould removing product - either an "Oxygen Bleach" type or now "eco" versions of these cleaners are available that attack the moss, algae and lichens and don't damage surrounding areas & gardens. If you use the Oxygen Bleach type, be prepared to protect surrounding garden & areas with plastic sheeting.
These products normally work by applying them to a clean surface, then leaving them to react before rinsing and brushing the product away with a stiff brush.
Note: many decking finishes include a "mildewcide", so by keeping your deck maintained every 6 - 12 months, you can often reduce or eliminate the problem.