I've done a bit of studying around and I think I've grasped the basics, but if you could walk me through more nuances of spray painting, I'd greatly appreciate it.
Objective: Spray paint on steel (and a little bit of plastic) for:
- semi-gloss finish
- high durability (chip, scratch, rust resistant)
- wet-cleanable (I think the semi-gloss coating should allow this)
My current procedure:
- Cover areas not to be painted with paper and tape.
- Scuff the surface down with 100-grit sandpaper. 
- Wipe clean.
- Prime. 
- Apply color coating. Let dry.
- Sand with 400-grit and apply another coat. 400-grit sand again. 
: How do I know when I'm done with this pre-processing step? Do I have to scuff and remove the manufacturer's coat until I see the bare metal? Or do I just remove a significant part of it? If you feel like you have time, can you name some reasons why I would want to do this? I imagine my primer/coating would stick better to metal than on top of another coating, but I'm not sure.
: How much priming is enough? Should I be picky about primer choice?
: I presume that this process gives the semi-glossy finish I want, but I don't see how multiple coat + fine sanding is going to achieve it. Will it really? Or do I finish with the color coating first, then apply a glossy enamel coat on top?