Have the same problem with my Kawa W and
with the CB. I took off the fenders of the W,.disolved
the rust and sealed them with Tectyl Amber.
That worked a while, but the rust returns, slower
than before but finds it way the to under sides.
The CB got a different treatment.
I put a lot of grrease on all inner parts of the fenders
and after some time it builds a hard combination
with the mud it has gathered, while riding the
bike. It's surely not a hundred percent protection,
but maybe it helps to keep the rust away from beeing
overactive.
But anyway, how old shall we become, to find out
out the year and the day, that the rust has worked
from the inside out? Hopefully I will be on this
planet for a lot more years to come, but I assume
to have fenders on my bikes, as long as they stay with
me, that are somehow rusted on the inside but
in a good enough shape to serve for what
they were build for.
So I try to see this way:
It's one of the cosmetic problems that the CB has
and there are quite some more with the 2013 serie.
In German there's a saying, ' Eins mehr oder weniger
macht den Kohl nicht fett.'
The CB is still an enyoable good bike.
Wisedrum