Why are there black spots on my deck after sanding?

[Dave-1]

Morning All

I have a question regarding what have I done or how come I have black spots on some of the just sanded and oiled timber deck?.

Story so far

I built the deck 12 years ago and it was oiled a year after I put it down, then oiled a year afterwards again. No maintenance since.

I sanded the deck back and realised that the timber was darkening in colour between the weekends lol So pushed myself to get it finished in a day and then oil it the next.

That rainy Monday evening we had was the same day I oile dthe deck. Started 6:30am and the recoated 1:30pm

The ideas I have where I went wrong potentially-

-I was sitting on the oiled deck while recoating and maybe stuff transfered off myjeans onto the deck?

-When I sanded the timber I also would sand the screw heads and maybe thats whats cause dthe dots?

-Possum/animal has walked across the beam directly above and shaken their fur? 

I didnt realise the spots werent just dirt until I was lying on the timber. They didnt appear in the first few days (I dont think)

They can be kinda scratched using my fingernail.

Its not across the whole deck only in certain areas and mostly the top photos (There is a beam directly above the area.

The sanded surface as I was putting down the first coat of natural cabbots oil.

After two coats of oil.

The next day after rain overnight and during the day. I was very happy with the result.

The decks condition before I started. It was generally this type of condition. Not rough to the feet, just super grey

Im not particulary fussed with the spots as the deck is outside and its undercoaver but open both ends to the weather. I was wondering if anyone has an idea on why it happened? I havnt really sanded a deck back before, only installed it new. 

I used a belt sander and did go hard on the sandpaper.

It was 36 grit sandpaper

Dave

[EricL]

Hi @Dave-1 

It's hard to tell what could have caused it. It might be long term mould growth or could be a chemical reaction on the surface of the decking panel. But the issue here is that it did not happen over your entire deck but only in certain sections. Plus, your deck is a few years old and therefore very well-seasoned.  

I suggest trying to sand it off and if comes back again then you definitely have a mystery on your hands.

Eric

[Dave-1]

Thank you @EricL 

Yeah, I like a good puzzle and trying to go backwards to figure what happened. I have considered sanding the sections and prob will once I do the pricacy screen that is next on the list to be sanded. Or knowing me I will use the deck for awhile and it will "blend" into the background chatter of everyday things that need to be done lol It actually might be interesting to sand and re-oil and see if it reappears mmmm

Thanks

Dave