scuffing the surface, and washing in soap/warm water helps. but it's really not absolutely needed ... as for a clearcoat, ultra flat clearcoat is available .. but again not really nesassary .. depends if you want that used/worn in look or you want it to remain utterly perfect and new looking.
the totally proper attention to all detail procedure varies person to person depending on their experiences... but my perferred method if it was a custom/show peice would be
completely disassemble, scuff down all the pieces being painted with 000 steel wool, wash all the peices in warm water with a bit of soap (just enough to clean, but too much will make getting all the soap off after washing some of a pain). then prime all the peices. wet sand @ 400 grit (water with a touch of soap) ... rinse off and prime/sand as needed for desired surface. apply colors/patterns/etc, wet sand, clear, wet sand, etc as needed (very dependant on what your doing)
for airsoft... typically I just disassemble, wipe or scuff as needed, tape if needed and blast it... beat on per usual and reapply if needed later on
generally I just do black so I use a good primer and be done with it ... if doing camo .. I blow it over the black, but I don't do anything very technical, if I want it that indepth I wrap it LOL