Boiler Flue staining

I have at home, in the garage, two gas boilers that vent outside to th
They work fine and heating is good in the house.
One small thing that I would like to fix:
On the outside wall, around one of the flue exhaust holes there ar
plenty of brown stains that spoil a large area of the white wall.
There are no marks around the other flue exhaust hole.
The stains are kind of brown or iron oxidized colour. They are lik
drops that when carried by the wind leave elongated stains of a radia
pattern around the lower half area of the hole.
These stains are very very difficult to remove or clean. I have had t
paint the wall twice because of them.
The Corgi engineer inspected the boilers and flues and gave them
resounding seal of approval, although at the time the wall was recentl
painted and nobody mentioned the staining issue.
I am guessing that one of the boilers is not burning the gas well an
the stains is some kind of residue.
It could also be some rust of the metal parts on the ouside wall aroun
the flue exhaust. There is indeed some metal oxidation there but i
Any ideas of what it could be and how to fix it?
Many thanks,
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