Before the Paint
This is what happens when bare metal sits out in the weather for a year or more, and it's a good argument for applying paint to your truck before rust begins.
The primary intent of this article is to cover paint (and other chemical) application. But the surface you apply the primer and paint to must be prepped correctly or you'll get even more practice when you have to reshoot everything. Most of the parts you'll paint on a truck are made of metal, so we'll focus on preparing it for paint. Some plastics such as the steering wheel, shown in this article, are readied for paint in the same manner as metal except that rust isn't a concern.
If your metal is already painted, you can sand the paint until it's smooth and then apply primer (if necessary) and paint over the top. If you need to start with bare metal, an epoxy primer should be used first to grip the metal well. Over this, a primer surfacer should be used and then paint can be applied. If it's a suspension or frame part you're painting, you can skip the multistep process and use Chassis (glossy) or Underhood (semiglossy) Black from The Eastwood Company that can be applied to bare metal or primer (but not over epoxy or etching primer).
We recently ran across a case of severe rust on an entire vintage Chevrolet body. Before paint can be applied to this, all of the rust must be removed and neutralized. For light-to-medium rust, a rust neutralizer, such as Corroless from Eastwood, is an easy, effective fix. This truck was severely rusted, however, and the entire body was affected. So Drezek Environmental Stripping Systems stripped it with a combination of sodium bicarbonate (primary ingredient of baking soda) and a slightly more aggressive medium. This procedure was able to lift out the rust from even the deepest pores of the metal. Then the body was thoroughly rinsed and shot with PPG DP90LF epoxy primer from Thompson PBE. For applications that aren't as rusty, Drezek can leave primer and body filler in place.
One of the problems with rust is that it's metal combined with oxygen: It's part of your truck's metal body. | 
Since the rust was so severe, Drezek Environmental Stripping Systems started by sanding the entire truck with 80-grit sandpaper. | 
Next, the truck was blasted with the sodium bicarbonate mixed with a slightly more aggressive medium. |