I agree with Dayclona on the old plastic, it is brittle and probably not worth the effort. But.......sometimes it's worth it, take my '68 Camaro SS grille........it was fairly new, but was broken and on Ebay for $15. It was broken from the lower corner to just under the running light. Easy fix, I ground troughs and valleys on the backside, and put clear tape on the front, to keep the glue from seeping too far. The glue penetrates all the way, yet doesn't turn into a hard to sand mess on the side you see. Then, I 80 grit with a DA, use regular bondo to smooth, and prime with etching primer. I have several repaired this way, and they haven't cracked in many years.