On Sat, 29 Nov 2014 18:21:58 +0000 (UTC), Adrian wrote:

I think my FIL does the same and he has never had a problem

Thanks for sharing that. It's reassuring to know that I wasn't
the
only one 'taking my chances' with this de-icing technique. :-)

Ditto that. Never had a problem with it.

Likewise here. Even on heavily-frozen windscreens with pre-existing
chips and cracks, I've never had them spread.

I guess the warning is for the brain dead who boil a kettle ful of
water take it straight outside a lob it overe the screen. Rather than
luke warm and carefully applied. Water straight form the cold tap
would proably work just as well. When it's getting close to or below
the ground frost point they water football pitches to keep 'em warm
and unfrozen.

The really nice side-effect is that it warms the glass, so helps reduce
misting on the inside.

We regulary get frost on the inside...

--
Cheers
Dave.