As for rust, that one is probably because the water mixed with coolant is full of sediments..
When I meant 'tarnish', I mean like your aluminium block, you notice that left long enough, it has that whitish spots you can scrape off. That's the tarnish I was talking about.
Rust will occur, because water + salty sediments is ever present, unless you use filtered water/RO water/etc.. but that's also not 100%.
Coolant is supposed to be mixed with water.. for me, I go with filtered water + coolant
I use the Motul Inugel Expert Ultra, premixed with filtered/RO water.
Some part of the instructions.
Do not use pure product. Mix with soft water according to the required freezing protection : refer to chart
To maintain anti corrosion properties, do not dilute below 33%.
Water for dilution : If possible use soft water : Maximum hardness 35°Fr and chlorides and sulfates
maximum contain 50 ppm.
As you can see, you can't dilute it too much, or it loses it's anti corrosion properties, and you HAVE TO use clean filtered water.
I've used it for the last 100,000kms, and the water's still blue in colour, the radiator is still clean, and I only needed topping up twice since I first filled it, the first time when the radiator wasn't full yet, and another, when I changed my fan switch and water leaked out (if you know how to do it without the coolant leaking out, let me know, I'm VERY INTERESTED :P)
So far, I've never topped it up since those two times, and the radiator cools pretty well, my temps are in the 80-90 range, and never exceeds 90 degs celcius.
