I want to make some one off ducts in Carbon, smooth on the inside for airflow.  I've made a rough male plug and about to skin it and apply pattern coat primer.

The primer would handle the resin fine, your issue will be crushing the plug. Don't underestimate how strong vacuum can be, if it was me I'd be putting at least 3 layers of 450g CSM on the plug to ensure it can handle the pressure. You will also have to envelope bag it, which with infusion inevitably leads to resin getting into places you don't want it to.