In order for fiberglass to bond. It needs rough surface, otherwise, It will just fall off and crack in the long run. Thisyould involve wire brush wheeling the modified areas where you will be using fiberglass. I would also suggest drilling many 1/8th inch holes for extra bond.
Too much work, Cough up more money and use urethane/plastic compatible epoxies... search for "SEM flex weld" on ebay!
and once again, I have a thread to promote the unvieling of my bumper project.