Hi Steve...
The problem you describe could be caused by dry spray or be caused by not tacking (wiping with a tack cloth) between coats in a poorly ventilated garage. The paint in suspension dries as soon as it hits the air and falls back on the car as dust. You then burry that dust under the next coat of paint and make a rough surface. This is especially true when not painting in a paint booth. Thus the importance of using the tack cloth between coats...
I am not going to comment too longly on duplicolor paints...its a low quality system, a very low quality system in fact. You should realy re-consider the paint choice for the exterior of the car. As for mixing, its not that complicated and the gang here will be more than happy to help you all along the way.
Now, even if its bad paint, you should still be able to lay a flat and wet coat of paint. I would practice on some scrap until you can adjust your gun and spraying speed to get a nice smooth coat. You can play with the gun's adjustments to get the finest even pattern of droplets from the gun. Atomization is key here. Sometimes it helps to close the material knob (the needle adjustment) a little and slow down the spraying speed.
Let the coat dry to the touch then use a tack cloth to remove all the overspray dust that will have settled on the car...
My 2 cents worth...
Serge