I understand primer is porous but what about acrylic matt black paint is it also gunna let moisture in?

No Acrylic is a top coat so will seal it fine as long as it is applied like a top coat not just dusted over but it also creates more work for yourself especially if your not driving the vehicle as long as it is out of the weather, covered with a car cover you'll get 6 months out of primer before you have to do it again.

Ill clear up my question, I want to paint a car matt black, using a proper top coat acrylic paint. Will it be porous? Mostly because I have never used matt black on a car.
I do not intend to paint it with black primer and leave it.

Mate don't go flat black,get your paint shop to make you some semi gloss.It looks cool, and its easier to keep clean.If you want Matt black it will cover your car ok,and if its a good quality pant it won't rust.

All matt paints are porous. This is how we make the paint matt, ie not reflective, ie it has a broken surface. The porosity is at micrscopic size. Shiny paints have a smoother surface finish and repel moisture.

As an experiment try rubbing some car polish over matt paint. The paint absorbs the polish so it is very difficult to rub off completely. The polish will tend to leave marks in the matt paint.