The art of a new paint job... How to?

Wet and dry is also called glass paper here, i would use primer if you are using white or a similar lighter colour paint as the dark green/ black shows thru easily but saying that i didnt on my white slim and it came out ok.
I just used cheap $2 spray cans from supercheap or autobarn, the gold one is why a primer and fine sanding is sometimes required - it was my first spray job and i wasnt really concerned with the finish.