Good old Plasticote gray auto primer from WallyWorld is a good primer for resin. Its also less expensive than a paint which is marketed toward modelers (~3 bux for a jumbo rattlecan)
Other good primers are American Traditions from Lowes and Painter's Choice from Home Depot. In fact, my finish paint color of choice for resin ship hulls is Lowe's American Tradition Rusty Red Primer.
Do wash the resin in soapy water and allow to air dry.