It is always best to sand it down to bare metal. But for a sub $2000 truck you can just sand/scuff the paint and the stuff will hold.

In my opinion, what makes a good paint job is not when you are painting, but all the prep work of sanding, priming, etc... There are lots of people on here that know way more about it than me. Remember if you see something in the primer/sanding stages, fix it. Putting a glossy paint on it will only highlight it.

I am assuming you have a decent paint gun?