for the back bumper, I sanded with 600 grit to get a nice tooth for the Self etching primer to bond to, then hit that with some 1000 grit to get a nice smooth finish and after that 3 coats of color, (body color is always nice) and 3 coats of clear. then buff to a shine. make sure to clean any wax or grease from the surface before starting. when it comes to paint, prep is the most important step.
the front bumper takes Alot of sanding to get rid of the textured finish, I used a 25 dollar jitterbug sander from walmart and some 600 grid and sanded them smooth then hit with primer and some 1000 grit and 3 coats of color 3 coats of clear. but there are other methods.
Take your time, and make sure the surface is completely clean and free of dust, wax and grease before proceeding.
If you want just a normal gloss black, for the front I would strongly suggest Krylon for plastics as it seems to be the most flexible and has held up well in the alabama sun for 2 years.
the first time I painted my bumper i did it in a hurry with walmart store brand paint and it turned dull and looked like crap after just 6 months.
USe high quality paint and Clear coat.
its also better to do light coats and build the color up, than it is to hit it all at once.
Take your time
I would like to suggest this video for a few pointers,