I would follow a mix of the two directions... first.. use the lexan directions and THOROUGHLY wash the exterior to remove release agents and such... but then I think you would be good with either future or black krylon as a primer... which is more the pastic side of things...
Good luck to you! I think you will love the stuff once you use it... if you have a spare model (as in a junk model) it might be good to try your hand at it first... not that it's any arder to put on than regular paint but knowing how it goes on helps...