Urethane resins require silicone or wax release agents to be used in the moulding process - some residue of this will inevitably be left on the finished castings. To ensure an even adhesion of primer & paint you should make sure to thoroughly degrease your resin pieces.
Water and washing detergent will normally be more than sufficient - if you have multiple castings to clean up, a bowl filled with soapy water will do the job; alternately an individual application of detergent and water, well rinsed, will ensure proper cleaning.
Make sure to scrub the surface and details of the resin casting - finger pressure will normally be enough.
Rinse off the soapy detergent thoroughly with clean water and set the resin aside to dry at room temperature for a couple of hours minimum (especially if you have sanded the resin - all resins are porous and will inevitably have absorbed some water).
Once resin is thoroughly clean & dry, it must be primed before painting. There are many suitable paint products available - we recommend Halfords spray primers as we've had long experience using them with our resins and find them to be consistently reliable.
ALWAYS use spray cans under conditions with adequate ventilation - ideally, outside. ALWAYS wear a mask or respirator.
Having properly shaken/mixed the aerosol can, spray the resin from multiple directions at a distance of 10-12" (depending on atmospheric conditions - paint will dry sooner in hot/dry conditions so you may need to move closer to avoid dustiness from dry paint). Depending on space/facilities, at the very least you should put down scrap paper/ newspaper to protect work surfaces from overspray.
A simple and effective spray booth can be made by using an old cardboard box (lined with paper, again) but be aware that paint fumes and excess spray will "bounce" back at you and may afect the evenness of your primer coat. Spray paint evenly and regularly side to side, across the whole job rather than concentrating in small areas (which will cauase paint flooding and potential loss of detail). Move around the job or turn the pieces around to ensure an even undercoat from all directions.
In an ideal world, spraying should be done under mechanically ventilated conditions - you can pick up a portable benchtop airbrush extraction unit for less than £50 online - the one illustrated was supplied with built in lights and turntable.
Leave primer coat to dry for at least an hour at room temperature before painting.