On something like a bumper, I think you're better off using something else besides putty. It would really depend on how the part fits and what the gap looks like. After puttying, you can't much remove parts, becauee most putty is solvent based so it sticks to the plastic better. If you removed it, it would breat apart between the car body and the bumper.
What you want to do is to dry fit quite a bit. Check the fit, see where gaps are, then sand, scrape, and file high areas away. Check the fit again and repeat until you have as good a fit as you can get. You can eliminate most gaps that way if you're careful and methodical, then use putty to fill in the small gaps that are left.
If it's too late for that, consider using styrene sheed or sticks or stretched sprue as a shim, you can then put putty in the small gaps that are left and sand that.
Madda
Trifles make perfection, but perfection is no trifle. -- Leonardo Da Vinci
Tact is for those who lack the wit for sarcasm.--maddafinga