oiled deck going black

[mongrell]

giday all finished my new deck bout a month ago its iron bark decking , problem is its starting to go black now whats causing this {fungal ?} and whats cure ?

[mic-d]

g'day mongrell, what did you oil it with? was it linseed/turps?

Cheers
Michael

[Big Shed]

Sure sounds like it...........

[mongrell]

stock standard oil form paint shop .just looked at tin ,manor hall its called and says its uv and mould resistant , didnt like it when i put it on layed on the timber ,ended up mopping it the get rid of excess , but i put this down to it being ironbark and so hard oil wouldnt soak in

[Big Shed]

mongrel, I'm not familiar with the product, it sounds as though it is a fungal problem. Do you know what sort of oil it is? I did a web search, Google, and couldn't find a reference to this product. Who makes it?
Even if it was pale boiled linseed oil based, one month is a short time for fungal attack to occur.

Is the deck in the shade for a long time each day. Is there vegetation around the deck that could drop leaves/resin to cause the black stain? Is the black stain all over, or only in places/patches?

Lots of questions I know, but it sounds a bit unusual.

If it is being spruiked as anti-fungal, I would contact the paint store in the first instance and get them and/or the manufacturer to come and have a look at it.

All else fails I would high pressure clean it off asap.

[strangerep]

finished my new deck bout a month ago its iron bark decking , problem is its starting to go black now whats causing this {fungal ?} and whats cure ? [...]
stock standard oil form paint shop .just looked at tin ,manor hall its called and says its uv and mould resistant , didnt like it when i put it on layed on the timber ,ended up mopping it the get rid of excess , but i put this down to it being ironbark and so hard oil wouldnt soak in

You said "bout a month ago", so I'll bet you didn't weather the deck
sufficiently before coating it. Iron bark has plenty of tannins and other
extractives that all need to come out before you apply any coating.
The black stuff you're seeing is just these extractives leaching up from
the timber under the action of the sun and concentrating underneath
the oil. I made exactly the same mistake myself, and got exactly the
same result (though my timber was turpentine, not iron bark).

Bottom line: all the extractives must be allowed to come out first.
Try this experiment: prepare a small quantity of concentrated Napisan
(sodium percarbonate) dissolved thoroughly in hot water. Apply to
a small section of the deck with scrubbing brush or scotchbrite pad.
Very quickly, you'll feel the surface go slippery - that means the
Napisan is starting to do its work. Squeeze out the pad in a separate
bucket and check how dirty the water is. I bet it'll be absolutely filthy.
That'll give you a better idea just how much tannins and extractives
need to get out of the timber.

I had to repeat this treatment 6 times(!) before I got it clean enough
(because I had mistakenly applied further coats of oil over the top).
This needs thorough hosing or heavy rain over the whole deck after
each treatment to remove the Napisan properly.
(I made use of the heavy rains in Sydney during
April/May this year to avoid water restrictions). You need to reach
a point that, when more rain goes on the deck, you don't see any
watermarks after it dries. Only then should oil be applied.

(I've written about this more than once before on this forum.
More information if you search back through my earlier posts.)

BTW, ironbark is an excellent choice - it's one of the most durable
timbers you can buy in this country, so don't worry at all about
whether it's rotting. It isn't. Ironbark lasts around 25 years buried
untreated in wet ground. It is extremely resistant to fungal attack,
and other things. All you need to do is gradually remove the existing
oil, clean it thoroughly, and start again. Oh, and steer clear of
high-pressure cleaners on timber except as a last resort. They can
too easily rip the fibres, after which you'll have to re-sand. See
how far you get with multiple applications of Napisan. That's all
I'd ever use now (followed of course by 1 treatment of Oxalic acid
to correct the timber pH).