I had very poor results with smoothprime, though some people seem to have had favorable results. Even so, I would strongly recommend against using it and would go so far as to suggest taking it off the bottom. On the other hand, if you have topcoat problems later on the bottom of the plane, it would be less noticable. But you will kick yourself for not taking off when you were at a stage when it was easy to do so. Im in the camp that finds that the only thing worse than sanding micro is repainting an Ez.