I've only painted one model built using castings, which was the Stuart Victoria. I chose to use Halfords spray primer (grey) and I think it was Vauxhall Pine Green + a gloss black.
I used abrasive tape 80 and 400 grade to get the castings smooth, and then sprayed two coats of grey primer. A week later when I could be sure the primer had hardened off I filled any remaining dinks with cellulose stopper. I then used wet & dry to rub down the piece and repeated the spraying and filling until the finish was at the standard I wanted. Final coat of primer, then I applied a coat of colour. Once that had hardened off (the following weekend) I gently rubbed down any bumps and bits without penetrating the primer coat. Couple of coats of colour and all is done.
The paintwork is four years old and is still good. But it is just on a shelf in my office, not standing any weather.
Steve