[Woodz428]

Water Wetter has been used for years in Road Racing Bikes. All the bikes I've maintained, it worked great in and the riders/owners loved it.
I have a little different opinion on the use of rain water though (unless you are talking about another product I am unaware of instead of the natural stuff). Rain water is full of all sorts of agents(elements) that can create corrosion and I think you could open yourself up for problems down the road. Any water I use in a cooling system is Distilled, it eliminates any introduction of corrosion causing elements into the cooling system.