what is it that you want to smooth mate? is it the rear bumper or the rear bumper into the rear quater?
some stuff that you will need are thing like plastic filler/ normal filler/ stopper. plastic parts primmer, two pack primer, base coat, laquer, spray guns, compressor, sanding blocks, d/a sander, flatting paper (grades from p60 up to p3000),
to be honest if you havent already got all that stuff, it will cost you more to buy it and do it youself than to get it done professionally. plus you will get a better end result.