One would think that using a very aggressive sandpaper will provide a rough surface for the paint to adhere to and be a better prep than using 320 grit. One would be wrong.
Know which grade of aluminum you are using, and use a primer designed to be applied over that aluminum.
Plastics can be split into two groups: polar and non-polar. Think of polar plastics like water and non-polar plastics like oil.
It all comes down to the properties of the metal or plastic, otherwise known as substrate.
There is an increased focus on safety systems for North American Class 6 to 8 trucks