Oiled Douglas fir planks become too dark.

Hello everyone,

maybe someone recognizes what problem I have here. I would be grateful for tips. I used to have a terrace made of Douglas fir - everything was great. The wood got darker every year when I applied new oil and slowly turned gray. Now, a few years ago, I had such a terrace installed in the new house after the WPC had deteriorated. I suspect it was inferior goods from the factory outlet. Anyway, the new boards discolor year after year. The wood is dark gray, blackish when wet. Only in some corners does the oil work as it should. Those areas are clean. So it can hardly be due to the oil, right? I have read that it could possibly be an iron reaction, but that seems quite widespread to me (29 out of 30 sqm). Attached are some pictures. I have now cleaned the boards with an electric brush - at least somewhat clean. What can I do to prevent it from happening again? Also, a few more pictures, this time in dry condition after cleaning. You can see the corners that are as they should be, and the rest...

Wood is a natural product. It can well be that it greys differently and also reacts differently to the oil. In my opinion, this has nothing to do with a defect, but solely and exclusively with nature.

Unfortunately, these are "crazy" floorboards with lots of grooves. I don't like them because they can't simply be sanded down like smooth wood. That's no consolation to you now, but otherwise I would have said, sand twice (medium and fine). But you will still have the problem with the grooves.

without having a real answer I want to point out: That does not look to me like normal graying or darkening of wood. The spots are somehow distributed independently of the wood grain. No idea if that is really the case and somehow helpful. Good luck with the [search for the cause].

Without having a real answer, I want to note:
To me, this does not look like the normal graying or darkening of wood. The spots are somehow distributed independently of the wood grain.
No idea if that’s really the case or if it helps in any way.
Good luck with the cause investigation.

Hello,
I think this is a misinterpretation. What you see in the pictures is the cleaned area. It’s not that simple, because some dark spots are "firmer" than other dark spots and I also didn’t want to damage the wood completely. In one picture, you see the completely dark beams – that’s before cleaning.
The iron test was negative.
To me, it looks like the oil darkened where the sun was blazing on it all day.
I have now cleaned the rest and am now using oil from another manufacturer. It’s actually almost too red for me, but okay. Let’s see how it develops.
When I removed it with the steam cleaner from the second section, you could see that the top layer … about 0.5mm … came off and the normal wood color appeared underneath.
Wood does gray over time, of course – but I had never seen it this dark before.
A bunk bed made of Douglas fir, which stands on the terrace, still has the same color as at the beginning, was treated with the same oil, but was a different type of wood.