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Black spots on new spotted gum deck

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  1. #1
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    Default Black spots on new spotted gum deck

    G'day all,
    I have recently finished building a spotted gum deck aprox 50m2.





    I am planning to use an oil based oil and am keen to oil it ASAP but I am unable to get rid of these black spots that quite a few of the boards have.





    After talking with my local trade paint shop and showing them photos we thought it was mould on the boards. The deck has been cleaned with Quantum Sap Strip which certainly dulled down the initial glazed look of the timber. It has also been cleaned with Flood Mould Action but this had no effect on the black spots. The spots do sand off but I was hoping to avoid sanding the whole deck. I am planning to do the napisan wash as described in this forum over the next couple of days.

    Does anyone have experience with marks/spots similar to this on spotted gum or other timber? Do you think the napisan wash will help? Would an oxalic acid wash help? Any advice much appreciated.

    Thanks

    Mick

  2. #2
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    I often see this spotting on new SG decking. As I sand everything first that I work on that would be my advice. A 60-80 grit sanding with a quality random orbital sander will take this off easily. The size of the deck in the photo could be easily sanded in 1-2 hours. After sanding blow-sweep- vacuum as much of the dust as possible off. Then just give it a good hosing down with fresh water to remove the last of the fine dust. I then blow dry the deck with an electric leaf blower and go straight to coating . However, I use a water based product. You are using oil so you will want to wait & make sure it is really dry. This will normally take a good day in the sun. Looks like there is some creature lurking on the deck !!

    Good luck

    jimj restore-a-deck.com.au

  3. #3
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    Thanks jimj for the response. When you say a quality random orbital sander do you mean a quality 1/3 sheet sander or are you talking about a larger professional type sander? Also, do you suggest sanding the whole deck or just the effected boards? Do you think I should still do the napisan wash?

    The creature is currently isolated from the deck!

    Thanks again, Mick

  4. #4
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    I guess what I meant to say is a 100 mm circular random orbital sander like perhaps Makita, Bosch, Milwaukee. These are all around 220-240 watts. This is not a lot of grunt but enough to sand these spots out with 60-80grit red discs. I would try 80 first. If a little too hard drop down to 60 remove the spots then go over with 80 to finish. These sanders are several hundred dollars but will give you a nice finish and a tool that can be used for many timber projects. I use a RO sander that is 720 watts but no need for that type of grunt nor the need to spend $900 either. After sanding just wash down & remove all dust. No chemicals whatsoever will be needed. I never use any chemicals in my preparation work regardless of the age of the timber. This idea that sodium percarbonate will extract the tannin out is true but you can never leach all of it out. I like to blow dry my decks to try & stop some tannin stain. If it dries with some tannin stain just sand them out with the 80 remove the dust with blowing-vacuum but no water. it is like removing chalk from a board . Easy with no sweat.

    Good luck & let us know how it all worked out. The paint stores love to seel you a 4 litre bottle of something for $50 that is worth about 50 cents. Take the $ and invest it into some quality beer!!!

  5. #5
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    Thanks again Jimj for the advice. I've gone and updated my sander with a new Bosch random orbital. Now to find some time to sand the deck. After its sanded and I hose it down is one good day in the sun enough drying time or should I leave it two?

    thanks

  6. #6
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    The drying time is something that will vary dependent on many factors. I help my drying by using an electric( not petrol) leaf blower. I have 2 Ryobis that cost around 99.00 that are around 2400watts. For blowing only they are totally blown away by the $49.00 Ozito from bunnings with only 1800 watts. The secret is the much smaller barrel. After I sand I then hose down with lots of fresh water then blow dry the deck. This also gets the water out of the countersink holes. I can then go straight to coating as I am using a water based product. If you at least blow dry the deck the sun will speed up the drying process. I would wait until you have forecasted 5 magic days ahead. Of course who knows when that might be.

    good luck

  7. #7
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    I had a lot of spots on my spotted gum decking. However it came off easily washing the oxalic acid and then the napsan. Wait 2 days for the deck to dry out and coat.

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    I wonder if this can be avoided by applying a mold killer straight after installation.
    Something like benzalkonium chloride, in 30 seconds?
    30 Seconds 5L Spray & Walk Away Concentrate | Bunnings Warehouse

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    I tend to add a product called VC175 which is a mould inhibitor-retardant to the stain. I would say at best it will reduce the mould by 50% which is better than nothing. Yes oxalic has its place and will move off black spots from metal filings and may shift these as well. I guess that I have been sanding for so long that I am used to it and can sand fairly fast. On new timber like this it is lightweight sanding and the sanding removes the darkening from the oxidation that has occurred. Ultimately the brighter the timber is made prior to the coating the nicer the result.

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