well what i would do is this:
1- try it your way, remove the bumper, but do it to the inside of the bumper if the inside has the same texture. if not...
2-try resin, resin up the bumper, then sand it down, add enough coats till when you sand it, the grit of the plastic grey bumper cannot be seen, then apply primer, sand that down then paint. voila...
let me know how it works out for you.