If trim is to be painted, automotive Bondo can be used to fill both screw and nail holes. It should be sanded flush after it cures.
One builder makes his own PVC plugs for screw holes during downtime.
A dab of PVC adhesive helps to hold the plug in place.
When all the plugs have been tapped in, a Japanese saw is used to saw them flush. Everything will be sanded before being painted.
Exterior-grade spackle can be used to fill fastener holes in PVC trim.
Before the spackle sets, wipe any residue off using a damp rag.
Once the trim is sanded, it can be painted with a high-quality self-priming 100% acrylic paint