Metallic needs good spray technique and gun set up to keep consistent and even, the lower lines of paint are even more difficult to lay even. If you have little or no spraying experience, then you would need to spend some time setting up your gun, and practicing so you get you r gun spraying well and even, you maintain even distance and proper overlap. I am not saying someone without much spraying experience can't shoot a metalic but not a piece of cake, and some colors can be a bear to keep from having a blotchy appearance. I would seal everything up before paint(assuming its a complete) with an appropriate color sealer of primer that will best help coverage of the paint. Don't spray overly heavy coats, as they will contribute to uneveness of metallic. If you can't get it to lay even with normal spraying well then a mist or drop coat should help even things out, or adding a touch more reducer. Do a search, drop coating has been covered before. Stir your paint well before painting, and you may also want to stir the paint between coats or agitate the gun occasionally. One thing painters use to do is keep some marbles or something in the cup and shake the gun occasionally, but that would not work with gravity feed guns which are most common today.
Metallics, everything really should be assembled and shot at the same time, or parts put in the position they will be installed and painted with the car. Variation in how heavy, reduction, temp ect can change how light or dark a metallic sprays.
I would plan on doing this with a basecoat and clear. A single stage metallic is even more difficult because you need to keep both a good wet glossy coat, and even metallic, and what you spray is what you get, cause you cannot sand and buff out any flaws in a single stage metallic.
Check your paints tech sheets for the grit they recommend for final sanding. Metallics usually need sanding with a finer final grit then solids. Don't use too fast a reducer or activator, even more so using a metallic.