Having no recent rattle can experience, I can't offer much there...
But if you have a way to spray quality automotive clear...
Start by cleaning the frame thoroughly with wax & grease remover. DO NOT sand or scuff!
Spray 2-3 coats of PPG 2042 clear and let it set for a week.
Now it should be hard enough to polish easily.
Wax it, build it, ride it, ENJOY!
Rattle cans only huhn? Weeell...
If the powdercoat is metallic or pearl color, your best bet is the automotive clear route.
If it's a solid color, you can carefully scuff the surface with fine Scothbrite pads.
I would then spray a coat or 2 of the rattle-can clear and let it sit for the 72 hours listed on the can.
Scuff again lightly and then apply decals.
Clear the whole thing with 3-4 coats to seal in the decals. That'll also give you enough film build to polish later. And I would wait at least a month before trying to polish. If you try to polish too soon, you'll likely just make it worse.