Thank you, Sandy. The white “threads” are in fact the cracks which have opened up in the black paint due to the expansion and contraction of the metal of the corrugated iron as the weather temperature fluctuates throughout the year. The relatively fresh exposed metal surface has not rusted yet in most of the cracks but in the last photo you can see where the oxidation process has started in the fissures.
Jessica's Nature Blog 
Like a forest of birch trees amongst an icy landscape!
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Thank you, Graeme.
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They have such delicate threads where I presume the paint has stayed. Lovely.
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Thank you, Sandy. The white “threads” are in fact the cracks which have opened up in the black paint due to the expansion and contraction of the metal of the corrugated iron as the weather temperature fluctuates throughout the year. The relatively fresh exposed metal surface has not rusted yet in most of the cracks but in the last photo you can see where the oxidation process has started in the fissures.
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Thank you for such a full answer. It is fascinating.
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