Winp®oClean
Try it for yourself mate get 2 buckets 1 with pure water and 1 with tap water wash 1 in pure water and 1 with tap water see wich one goes milky!
Both mate :

It's oxidized paint that that mixes with the water, any water. You end up with almost a very weak paint mixture!
This is what causes the "milking"
Dude, I've been wfp'ing these kinds of windows for six years, estate mansions, farm houses etc all with very old, oxidized frames. I've come across them all.
In a lab' situation the pure water might absorb the paint more but when it's being scrubbed with a brush it don't matter what liquid you use it's gonna milk!!!!
