No, the most misleading thing everyone hears about paint jobs is that your existing paint must be sanded down and primed in. Thats incorrect. The best Primer you have is your original paint job (thats if your existing paint isnt peeling off the car) The only time you need to prime anything is in or take it down to the bare metal is if you had a sticker somewhere previously that you dont plan on replacing later(it will bleed back through if you dont sand it do the bare metal, and feather edge it back then prime it in), Bare metal showing, or anywhere you did body work.

Many shops are different, everyone uses different paints which require different prep work. Ive sprayed them all, but i love the way DuPont sprays/wetsands/buffs. What i do, is use a DA and sand the existing paint job down with 320. Anything higher (400+) would work just fine as well but would take more time. DuPonts Sealer recommends sanding it down with 320 due to the fact that anything coarser the sealer wont fill the scratches in and you'll see DA marks/swirls all in your paint work. Make sure you sand around the edges or your door handles, window moldings, etc etc because if the paints going to peel anywhere, it will be along those areas.