DaveO -

Glycerine is a bit like the silicone oil that appears to be the "active ingredient" in the RainAway and similar formulations. Once you put it on a clean piece of glass, it doesn't matter what the directions are - it's very difficult to remove all of it.

That is, with glycerine it's difficult. Some say with silicone it's impossible, although there are ways ... (some of which may require a license to use).

John