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I am not bothered by it, and I
like the spotted gum appearance.Can you advise if such black areas are normal for the spotted gum, something to worry about, or considered a defect?
Photos attached,
Hello @sabramohamed201
Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about spotted gum.
The black markings on your spotted gum boards are natural and part of the wood's unique character. Spotted gum often has dark streaks or patterns due to its high tannin content and natural grain variation, so these markings aren’t a defect or something to worry about. They can add a rich, distinct look to the timber, which many people appreciate in furniture or decking.
However, if you prefer a more uniform look, you could hand-select boards or lightly sand and finish them to soften the contrast. The store has products like Cabot’s Decking Oil or Feast Watson Clear Varnish, which can help bring out the wood’s natural beauty while protecting it, making the darker areas blend more seamlessly with the rest of the timber.
Let me call on our experienced members @Dave-1 and @Nailbag for their recommendations.
Here are some handy step-by-step guides:
If you need further assistance, please let us know.
Eric
As with @EricL comments, Spotted Gum is a beautifully grained species that features that uniquely black grain running through it. My Alfresco area has a spotted gum deck and I have also used thicker 45mm pieces as feature shelving and even our laundry bench top. In doing so here are some things to consider that I have learned over the past few years. I have supplied a photo also of new floating floorboards with that genuine SG markings and grain.
Other than Bunnings who don't stock SG, no other timber suppliers will let you pick and choose lengths and boards. When ordering decking, you get what you get. You can only return boards severely damaged or disfigured. They will allow individual boards when only buying a few lengths to replace old damaged ones, but even then they don't like to, especially if they have to break a pack.
As timber mills close around the country, it's getting harder and more expensive to find "Genuine SG". "Tallow Wood" is often sold as SG as it looks very similar. But it has little to none of the black grain and no sign of black borers which are a battle that have eaten into the tree during growth. This is how you know it's genuine SG. So, don't buy anything over the phone, you must see it what you're getting in the yard. I have supplied a few photos of my deck boards where you can see some areas are slightly hollowed out from the borers, but I like the look for our coastal styled home. Buts it wouldn't be a reason to return a board either if you didn't like that.
Unlike say Merbau thats constant in finish no matter where and width you buy is consistent, this doesn't apply to SG. It can vary in widths depending on the mill supplying the retailer and the colour can be vastly different. Again only an issue when replacing boards which I have already had to do.
As a decking board, I have not been impressed with its durability against the summer sun. Despite using 14Ga batten screws and multilayers of a high-grade decking oil, I had quite a few boards cup, crack and seperate at the finger joins. I had avoiding replacing them because of the expense, availability and the noticeable colour patching compared to the undamaged boards. The replacement boards I could only source were 5mm wider and had to be planed down. Being hard to source I bought a few extra as spares. Last Dec SG was around $15/Lm against Merbau which is around $9/Lm.
If I had my choice again I would go Merbau. But for a much nicer grain finish and one better suited for direct hot sun, it would be Blackbutt. But thats even dearer again.
Nailbag
Good Morning @sabramohamed201
Wooooo I really like @EricL and @Nailbag 's explanation. I cant really add to it other then personally looking at the timbers before you buy it.
One thing I can add tho is what NOT to do 
I have merbau, a white gum? (bought from bunnings but think its discontinued) Jarrah I bought from a timbermill. When I have sanded the boards later on I realised the scre heads were flush with the boards. So when I sanded the boards tiny pieces of metal were scattered over the deck. I washed the deck and swept it clean and then oiled it. Within three days I had areas of black spots appeearing all over the decking.... 
After much thinking and talking to Bunnings in store and in here Ive worked out it was the steel from the screws being sanded that reacted with the oil and cused the spots. Good news they fade! Bad news I then used a grinder to cut some steel wire for a gabion basket on the new bench and guess what, several days later that black mold appeared lol Yeah kicked myself for not thinking it through and yes I did sweep and clean the area 100%
So whatever boards you end up purchasing, future proof them by making suire that the screws are countersunk and below the height of the timber is my suggestion.
The question I raised Why are there black spots on my deck after sanding?
Dave
Evening @Nailbag
100% agree with you on the countersinking tool. I used one but as never having done a deck before this one I thought the screwhead would be best level with the timber Its stood the test of time at around 12 years without any issue other then the sanding and that dissapeard after a month. Experience tells me now to have the screw a little deeper to allow for the sanding but back then it was my first deck with screws.
Dave
Morning @Nailbag
Now that is worth noting! 
Thank you!
Happy with the work I did but something like the sander you describe would have been perfect!
Dave
Now, the decking is completed, it was worth the effort - look really great!
what is the next step? cleaning and coating? can anyone provide recommendations please,
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