Maybe it depends on the brand of paint you use, I have used Tremclad (konown in the US as Rustoleum, I believe) after reading an article in HotRod Mag and it works quite well:
I have sponged a '70 Impala (big car!) with flat black, 2 coats give an excellent finish;
I have rolled semi-gloss white on grill/bumper of my '57 Suburban, it does not cover as well as flat black, but gives quite a nice, satin finish, after 3 coats.
I remember HotRod Mag thinning their gloss white a lot (consistency of 2% milk), rolling a lot of coats (8 or 10?) and sanding for a nice finish.
Remember that this a CHEAP paint job, it will never win a show, but can be quite presentable. I certainly will do it again.
