Inq, I know the look you're trying to achieve.
Start with a black basecoat. (spray primer whatever)
Then, get flat black spray paint.
hold it about 12 inches or so from the model.
(yeah yeah, mask of anything you don't want to have the cast iron look,...)
Now, hold the spray nozzle in, but keep it pretty far from the model. only let the tip of the "cloud" of spray hit the model. (this should put only the lightest of paint coverage over the black primer base. THis will give the 'slightly bumpy" look of Cast iron. I know it doesn't look too much like it yet...read on.
Once you can physically see the little minute bumps of the black spray paint, and it looks pretty good, let it dry.
Once dry, water down some regular black paint 3-1 ratrio paint-water then go over the model brushing it on.
THis will add the "petina" of cast iron THe paint will seal over and give the black solidness of the Cast Iron, while the small uneven underspray will give it the thickness and feel of cast iron
***BEWARE don't go nuts with these layers. too much will be ugly..real ugly. try this out on a sheet of sytrene or even an empty blister packet so you can see what i'm talking about.***
another way to do this is hold the can 6-8 inches away and triple tap the nozzle, it'll build up air between the paint com,ing out and it'll give the bump effect also, remember, too much....is not a good thing.