First off, if you can't be bothered rubbing it back then I wouldn't bother painting it at all, get someone to do it professionally. Spray painting isn't something you should rush
Ok, considering it has already been painted, you don't need primer. If it has any scratches or dents that you want to fix then get some filler, filler acts as primer so you don't need to prime it again. If you are filling it, sand the bumper back with 240 grit, then spray on the filler. Once that has dried, sand most of it back with the 240 until you start to wear through to the paint underneath. Then go over it with 400 grit until it is all completely smooth. Make sure it is perfect, the paint won't hide anything.
Now you apply the base coat. Do not wait for it to dry before you apply the clear coat. You should apply the clear coat within 15 minutes, otherwise they will not chemically bond and the clear coat will peel off.
If you hit it perfectly straight out of the gun, then there shouldn't be any need to sand it back. Just buff it with some cutting compound to bring it up really smooth, and you're done Don't wax or polish the paint for 30 days, the paint really needs to set before you do.
*EDIT* Also, if you are painting in this cold weather, you need to warm the bumper up. You shouldn't paint anything that is under 22 degrees, otherwise the paint will not shine and will come out matte. Just get a heat lamp or stick the bumper over a heater until you can feel that it is quite warm to the touch. Don't forget to wipe it down before you spray it, get rid of any dust and grease from your skin if you touched it.