To get your plastics to be smooth like the rest of the car you need a qaulity primer such as a 4-1 2k based grey primer. Now this stuff can only be painted from a spray gun. It's a remarkable product if you have the equipment to paint it.
Now as already gone over but for the super smooth finish if you want it.
spec IV touched on most of it.
Before applying any paint you should get yourself some wax@grease remover which will bond the paint alot better. To get a smooth finish from a can, i buy some spray putty which is mustard in colour. Then apply a primer grey over the top. When painting plastics it is recommended that you use a plastic primer as is still alows the plastic to bend. Now sanding is key too but not to much say some 600-800 grit paper before you apply a top coat.
Colour cans are of pretty good qaulity nowadays and can give you a pretty decent job as long as you take time. Dont forget your clear coat though to give you a nice deep shine.
Clear coat is the hardest of paint to apply and i have some tips. GO SLOW keep the can moving in one dirrection and overlap your strokes by 50% going one way then come back overlaping 50% the other. This will give you depth and also u wont end up with dry or "orange peel" in your paint job.
Make sure when painting take your time and make sure the temp is up. Too cold and the paint will do all different things to you. Do it once and properly and you will save yourself heaps. You live you learn though. ENJOY
