Doug,

This is one of those topics that everyone will have a slightly differing opinion and approach.

I routinely grit blast my resin models, wash them and let them air dry before spraying a grey primer on them. For plastic kits, if I've made a lot of modifications (i.e. filled holes, added rivets, removed rivets, repair surface defects, etc.) I still usually give these cars a light grit blasting; if it is a car that really didn't require those types of modifications, I have been wiping them down with a cotton ball soaked in isopropyl alcohol; I then let the car air dry, and apply a grey paint to them. I think the grey paint will give your finish coat a better "color."

Regardless, I always grit blast the plastic trucks; you can mask off the holes for the axles, or otherwise plug them to prevent the grit from getting in there; I usually just blast the truck, as is, then put them in a jar of water with a drop of detergent and stick that in my ultrasonic cleaner. This usually gets all the grit out of those tiny areas and recesses on the trucks; I then use the tool sold by ReBoxx to ream out the axle bearing holes. I think grit blasting the plastic trucks gives a great weathered look for our steam era freight cars.

YMMV

Jim Brewer
Glenwood MD