Try an acrylic paint for the first layer, then anything else will work on top of that, including Scalecoat and Floquil (or more acrylic). Acrylic seems to bond to the urethane whereas lacquer-based paints don't.
Jim King President, Smoky Mountain Model Works, Inc. Ph. (828) 777-5619 <www.smokymountainmodelworks.com> Trainmaster, Craggy Mountain Line RR <www.craggymountainline.com>
