Your questions are actually answered in the thread that you linked.
In a nutshell. Fusion paint isn't primer. It's a base coat color available in gloss and a satin finish - neither of these finishes are ideal for primers. I personally haven't seen any color of the Fusion line available as a true flat paint that has a decent tooth to it.
I'm currently using Krylon Primer to prime the bare metal or plastic, then Krylon Fusion Sunbeam Yellow for my base coat. I then need to let these dry
AT LEAST over night and then seal with Testors Dull Coat. If you don't dry at least over night the Dull coat and craze and crackle as it dries at a different rate than the spray paint it's laid on top of.