I think it would be a lot safer to do in the car, but the cleaning up after will be a bit of a job as it kind of goes everywhere.
On the car I did a water rinse and a towel cleaned it off easily, just took some time to get it out of the nooks and crannies.
I used a variable speed drill wide open and a spray bottle of water to keep the compound wet.
The water also helps keep the glass cooler also, since you don't want a lot of heat buildup localized on the glass or it could shatter.
