Hi,

I tried with my polarizing filter and it turned a tad better.
However, the sky was still ooverexposed ;-( I can't try again now
because it's overcast... we're getting rain soon ...

I don't think any filter will help another way you couldn't achieve
using only your camera. You just need less light, that is a
smaller aperture or a shorter shutter time.

I suggest next time you give it a try with various exposure settings,
ranging from -2 ... +2 compensation and take images simultaneously as
jpeg and RAW. Then have a look on the computer and judge which is best
so you'll know for the next time which way to go. After all: taking an
image more doesn't cost a single cent. Just be sure to delete the
unworthy pictures.

From my experience there is a lot more to raw data than jpeg, especially
if the light situation is difficult: you can safely alter exposure by
+/-2 levels on the computer. The backdraw is that it requires a lot of
time afterwards.

Best regards,
Ingoo