Please help. I am trying to paint door and window casings/moldings. They are old plastic brown that I want to update to white. What can I use to "seal" or prep?

Try the Glidden Gripper Primer. That stuff works on basically anything. I've never tried in on moldings but I know you can paint it on brick, walls, textured ceilings, ceramic tile, and walls and never have and peeling, cracking or other problems!