you cant use those paints together.for a base coat all you need is a LIGHT coat of basic primer, i use the cheapest kind from walmart (the blue cans on the bottom shelf usually) or krylon primer if i can find it, but its pretty rare as they have that dual stuff now. the ultra cover you used has an additive in it, i forget what it was, but it interferes with the enamel in the top coat. ive been using spraypaints for many many years, all kinds for cars to model kits to go cart bodies etc. wetsand it down with 400, apply a light coat of cheap primer, then top off with the rustoleum professional.
its also important when you're shooting the professional, the stuff comes out fast, so do 3 light coats, waiting 5 mins between each coat. the dome is a bit more difficult since you have to keep turning it to ensure the whole thing is even. the wet coats blend into eachother which gives it the smooth shine. if its smooth and doesnt orange peel, let it dry for 5 days. i left mine in a cardboard box with a bunch of silica gel packets you get from shoe boxes etc.
my scootch build here should help you out:
http://forum.501stspecopsdet.net/index.php?/topic/2112-dizzys-cap-w-bucket-restoratation-and-build/