What is the proper procedure for spray painting metal and plastic? [closed]

Question

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. [1]
• Wipe clean.
• Prime. [2]
• Apply color coating. Let dry.
• Sand with 400-grit and apply another coat. 400-grit sand again. [3]
• ???
• Finished.

[1]: 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.

[2]: How much priming is enough? Should I be picky about primer choice?

[3]: 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?

Answer 1

I did spraypaint a computer case a few years ago. It was glossy and I could see my reflection in it, but not as scratch-resistant as I hoped.

Question 1: You need to scuff to remove the smoothness of the steel and plastic to give the primer a surface to stick to. No need to go back to the bare metal.

Question 2: It depends how smooth you want your final result to be. Your end-result will be as good as your priming job. Prime, wet-sand with 1000grit paper, dry, repeat. I did this until I had removed all the "orange peel" feeling to my surface. (Took 6 or 7 coats) I used store-brand automotive primer and paint.

Question 3: This process give the smooth finish. The more coats/sanding you do, the smoother and glossier it will be. I used 1000grit for the first 3 coats, and then 2000grit for another 3 coats of paint.

The ??? step:

• Apply a few coats of clear coating. That's what I didn't do back then.

• When dry, polish it with car wax.