From an automotive finish stand point, metallic is very hard to do correct. Its best to use an air HVLP gun for metallic. Metallic spray paints have the most aerosol propellant per volume of any spray paint, because of the weight of the metallic specs. (Hence why huffers who sniff spray paint buy metallic). Using a can sucks. Period. Its not really intended for a huge area without looking like garbage. If you are spraying with the panel vertical, the metallic will go down with gravity to the wettest portion of the paint. Different paint requires different technique, but you have to factor in the air drying the paint before it hits the target. In the automotive world its air pressure. To much causes a cloudy overspray look, because the paint is being pushed with so much air its forcing it to begin to dry midstream. Easiest way to explain. Spray to close and it will run. Yours looks like overspray from being to far away. I don't know how much you have left to do a test area, but try this. Take a can and do a uniform spray from left to right. Start off the left edge and continue till you go off the right edge. Over lap by half when going from right to left, again begin spraying before you touch the target and stop after you leave it. Do maybe two feet top to bottom like this, do not wait for it to fully dry. Give it like five minutes and go again. See if you can get the paint to mold together before it has a chance to dry and cover evenly.
If you are dead set on metallic, go to Harbor Freight (or whatever your equivalent is) and get their gravity fed spray gun, very easy to use.
http://www.harborfreight.com/20-oz-high-volume-low-pressure-gravity-feed-spray-gun-47016.html <=- this has no regulator attached, if you need one there is a model with one. Or buy an external.
Go to your local automotive paint supplier and get the color you want. They will advise you whats best and if you need additives. Dont buy the most expensive paint, you arent doing a show car. Personally for this I would get single stage, it contains your base coat(the metallic black) and top coat (clear) in one. (If they do that there) You will have a reducer to add in to it. When you add the thinner or reducer, it doubles your total quantity. So a half gallon will turn into one whole gallon in the end. You will lose some with air, so half gallon should be good. Use two good primer coats for the paint to bond to, wipe off excess dust or dried primer dust from being airborn with a rag and acetone, and begin to spray. Wait 24 hours, and your done. You can wet sand with 1500 grit sand paper and buff with compound if you want too. Make sure to put acetone in your paint cup of the gun before and after spraying to get any contaminents out. You can also adjust your spray pattern (fan) before painting. Spray about 12-15 inches away from your target, using the same method I told you for using a can. This route you could do the left side, then do the right side, go back to the left side and back to the right side and you should be good. The clear will float to the top. This may cost you with paint and gun $80. Sounds like a lot, but metallic spray cans are what $5 a pop? And turns out like that.
Rolling metallic for the most part, does do what was mentioned. It wont lay easily and evenly off a roller. The plus of using the automotive paint is you can choose a metallic color from what ever make of cars you want. What ever you pick, do not use GM/Chevy/Pontiac/GMC. Their black is actually a brown when hit with a flash light at night. Its not really black. Understand though, the more expensive the manufacturer the paint will follow. So Mercedes color paint will be more expensive because the cheaper brand may not have the available ingredients required to produce that color that the expensive end does. As long as you do a good job using primer as your base, it should adhere. Im not a fan of using filler primer, its intended for spots not an entire area.
EDIT: **WARNING** If you use an HVLP gun to paint with, where a respirator or mask with filters. Not those dust masks you see the Chinese wearing in every photo. Harbor freight has a decent one that will suffice for about $15. You will die from fumes if not.
http://www.harborfreight.com/p95-maintenance-free-dual-cartridge-respirator-66554.html <=- this will give you about five different times or painting worth of clean air before the filters are clogged. Works great for sanding with an electric/air orbital too.