you also need to ensure the metallic remains in suspension. Mix well before spraying and spray immediately. A few marbles in the pot (if using a siphon gun), and keep agitating. If using a gravity gun just mix well and spray.
Your base (color) coats should be pretty light, you want very little flowout because the metallic will start to separate from the paint and render a totally unacceptable look. Even rattle cans can show this 'effect'.