the weak milk spray is a solution for powdery mildew, you need to use fat free milk and mix it with water, not sure on the amounts but its all over the net, so if you ever have any issues with powder mildew (pm) then you can use this weak milk spray to get rid of the pm, works wonders, its what most growers use when they have problems like that, as GiGa said, its also a great foliar feed as well, it can be sprayed on the buds and wont cause any problems what so ever, ive never tried it myself but if i had an out break of pm then id use it for sure, just make sure you use the fat free milk and mix it to how it says then you can spray it all over the plant, but make sure its not done when you are under the lights, its best to move the affected plant out of the grow room and then spray it and let it totally dry before going back under the lights, this will stop and leaf burn.
but yep a weak milk solution is a great foliar feed but its only really used when you are getting problems with pm, i may even be using it myself in the next few days after what i found on top of my soil today, i just hope i caught it in time, but if i have not then ill be using the milk spray for my plants and this will get rid of the pm, its suppose to work better than shop bought products for pm,