You only need primer / undercoat IF you plan to paint color such as (red,blue,yellow,or normal color) depends on the original color of the surface.
You MUST apply primer / undercoat IF you plan to paint color like (metallic,pearl,candy,etc)

Clear coat / transparent paint is not so recommended bro.If will just ruin you paint as it will not really gives a gloss effect.UNLESS you use a great cleat coat that is using a gun-spray paint.

Its kind of hard for you to remove the blue paint without removing the color below it.The paint already sticks to each other.
Why don't you just sand all over again and start fresh.