I've just discovered this post so sorry to bring back an oldy back.
I had a glass fuse' from a 2x2 primed and painted (white only) at a car shop.
It added just over 800g!
To say I was reather pissed would be an understatement... all the shop had to say was: "Just look at that finish!"

Indeed it was a finish Rolls Royce would have been proud of.
Specially that last WET coat!