Firstly, remember a rusty car won't be glossy!
This old pickup was sprayed in a Tamiya military flat, with a lick of semigloss clear. You don't want it totally flat.
A few rust holes were drilled through the panels here and there (I shouldn't have left them round - my mistake!); the rest is done with Humbrol enamels. Not easy to get in the US I understand, but the commonest hobby paint in Australia.
Military modellers have an amazing variety of paints, powders and washes available.
AK Interactive is a good brand I've used. If it works on a tank, it can work on a car. Just don't overdo it!
The parts that look as though the paint is worn thin - that's done by rubbing graphite (used for lubrication locks) onto the paint with a tissue.
There's more pictures at the link.