Rendered with Blender Cycles 2.79.
Used Denoise at default settings.
Samples set at 7000.
Fire flies on metal.
Strange shadows look to fine or to thin.
Hardware used is 2 x RX 480.

Disabled Multiple Importance and fireflies are gone but now the walls are blotchy.

Maybe instead of disabling multiple importance try enabling clamping in render/sampling? You can also change shadows with size of the sun lamp.

Thank You patwork,
It works fine now by using clamping.
That reminded me of seeing that with the Cycles Classroom File.
They used a value between 3 and 5.5 and it work for me also.

I used an hdri called “flower road 8k.hdr” and you need to find that on the net.
Here is the Cycles Blender File:

I think it looks pretty cool. It’s hard to see, but are you using a displacement modifier for the rust material?

No, I there is no displacement modifier in the rust material.
You can get the file for free from here: