[roberto]

it all depends on what he means by (a bit crusty)
that could be small or a lot depending on the person looking at it
they usually rust heavily on all the welds

taking it off is the best long term solution (MJ project for powder coating)
or this is what i did on my A6

i used a 4" twist knot wire brush on my hand grinder
i bought several wire brush wheels that fit in my drill
and hand wire brushes

fetling away for hours
then using a toothbrush to get in all the awkward areas i applied rust killer (hammerite kurust) then i painted with hammerite paint and lastly waxoyled inside and out..

4 years on it still looks very good

you need a ramp and a very dry garage too

sadly most audis suffer from this even my friends 2015 A6 C7 is now Very Crusty but hes not bothered like most people they cant see it and probably never will..