Aim the flow of air at the intersection of the rod and the plastic you are welding. Be sure to melt both the rod AND the existing plastic. After laying down the plastic, melt away the excess rod with the side of the hot air welding tip.
If you are using a nitrogen welder (which gives a stronger weld), go ahead and switch back to air. (This will help you conserve your nitrogen.) Otherwise, go to the next step
Use the airless welder to smooth out any curled edges that often occur while welding.
As you can see, this is a quality repair that saves over having to buy a replacement bumper cover