One method of covering balsa with plastic is to seal it at the edges and then shrink it with a heat gun. If you try to iron it down you will get wrinkles and bubbles. Most directions for covering sheeted surfaces advise ironing from the center out. Be sure you have plenty sealer on the wood or the grain will surely show through the plastic no matter the brand. Wood should be sealed just as well for covering with plastic as it is for painting. After going to all that trouble it's almost as simple to paint it after it's prepared.