Help!

I have a large house that's three years old. It is coming up to the
time when I need to do something drastic to preserve the walls which
vary from grey to light brown. I understand from the builder that he
put Sadolin Classic on it some time ago. But another coat of something
is needed.

I've spoken to Sadolin and they say that I would have to sand out all
the grey and apply a woodstain. I prefer my grey house and just
wanted to put a clear preservative on it (and don't want to have to
sand by hand 3000m2, particularly due to the fact it's a two storey
house.

Anyone got any bright ideas? Ideally I want to just brush down the
grey wood and spray on a preservative.

many thanks for your help.
Jon