On 25/09/2019 10:29, Primrose wrote:
We painted some of our fence panels last year, today I noticed that
there are rusty coloured growths on the painted panels which are
slightly raised.

Does anyone know what this is please?

Sounds like the spore bodies of a fungal wood rot or it might be the
beginnings of lichen growing on the surface. A picture would help.

Lichens are usually pale grey or greenish and slow growing.

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Regards,
Martin Brown