I have done body work all my life. All I use to cover glass when grinding around it, is to cover it with regular, thin, cheap, masking paper.
I know it sounds like it won't work, and or, you may think that it will catch fire. It does seem impossible to suggest that a spark which is hot enough to stick to the glass, is repelled by a piece of paper. This is one of those things that sounds like it won't work, but it does. I have done this a million times.
Perhaps the way it works is it provides enough of a barrier to slow down the spark a fraction of a second, so that it is cool enough it can't stick to the glass? :confused3:
It is obviously not the best way to protect glass from welding sparks. But, it will in fact also work for mig welding sparks, provided you are not welding in close proximity to it. You can successfully use paper to cover glass that may be at risk, if you can't get all of it with your welding blanket.
3M actually makes a real nice self adhesive paper to stick on glass, for this purpose. But, like most of their products it's pretty expensive.