That 'Wynns Industrial Adhesive' stuff is very good on fairings,.. as long as you have all the bits obviously! Like Yamahama said, you don't need to use the activator, but if you do, don't use the brush, because it can waste the rest of the activator in the bottle, use the aerosol thing.
It also helps to cut a real shallow 'V' shaped groove in the back face of the plastic, along the crack, just to get a bit deeper glue depth.