While it`s much easier to keep it off the surrounding areas, and I do think that`s generally a good thing to at least try with most products, I get the best results when I spray directly on the glass, especially with Glass Cleaners that foam up. Easier to get an effective (or at least effectively long) Dwell too. I wish it were otherwise as there are some areas I can`t do that way (so every time I do some one way and others the other way, giving me an ongoing direct comparison between the two methods). Sometimes the diff is minimal, but other times it`s quite significant. Apparently it`s another of those YMMV things.