RickB said:
So do you need to remove the paint first or just 'rough it up' a bit?
Always remove the old paint down to the bare metal before applying body filler. Preferably with a 36 grit disk. If you put it over paint, it can only adhere as well as the paint. You will not know until it blisters or cracks later. Also allow it to age for a few days before final primer and finishing, because it can shrink. Never thicker than 3/32". We've all seen it done wrong and that's why it has a bad reputation. Done properly, it will last as long as car. I've put in more than a just a gallon or two in my lifetime.
I do not like the Bondo brand, Rage is my favorite, but it's been around awhile, so there may be some better ones around.