what i find best is a hard surface cleaner called "powerfull" i don't think they had a very good design team, but it does cut through anything, and when i mean anything i do mean anything, will stain the glass if left on too long, but all you need to do is spray and scrub with the brush, ive found it gets anything off and you don't need to work hard,
but with any chemical always wear a protective glove, a thick one, otherwise your hands dry out and doesn't fee too good when your hands start to crack up.