Sealer Neccessary???

[Mtbr41]

I am thinking about painting my cart, it is plastic, no scratches etc. It will be flat black, I already have a gallon, of AUTO Paint Pro paint, just wondering if I need a sealer for the plastic?I am thinking about 2-3 coats of primer, followed by 3-4 coats of black....I want it to look good, but sealer is freakin expensive.

Another question...should I wet sand between primer coats and black coats, or just sand/scuff?

Also, what is the purpose of wet sanding?

Thanks, and just in advance I have been reading other posts but there are many different answers...