First question - why are you going for a full squirt? Are you changing the colour or staying with the original colour? I would strongly counsel against doing a major colour change, because then you absolutely have to take off door trims, paint inside doors and door shuts, under bonnet and boot etc. Not worth it unless you have plenty bucks to spend and want to have a show quality car
As previously advised, take off all trim, side moulds, lights, etc - otherwise you have to mask these off and you get paint edges which are a total give-away that you've repainted. Basically unless there is damage which needs repairing before painting (dents, deep scratches etc) then all you need to do is give the original paint a really good scuff with a Scotch-brite pad dipped in flatting paste like Scuff-it. Don't use "sand-paper". If you have to use abrasive papers use wet&dry carborundum papers no coarser than 400 grit. This provides the new paint with a good key to ensure no delamination down the track. An orbital air sander takes a lot of the hard work out of the job.
Don't sand back to bare metal unless the original paint is delaminating or really poor quality with cracking or oxidations going back to the primer coat.
You have to be using an angle grinder to get past bare metal and really **** up.
After flatting back every panel, use a product like Prep-Sol (wax & grease remover) with plenty of clean cotton rags to remove all surface contaminants. Assuming you're not painting the car yourself, your panel shop should use a mid-coat adehesion promoter like DuPont's 222S - it's like double-sided tape for paint, which chemically bonds to the old paint and provides a good key for the new paint. Unless you have areas which have been repaired using body filler, or have rubbed back to bare metal, generally there is no need to use a primer.
However, if you are determined to apply a primer coat yourself, go for several light dry coats rather than a heavy wet coat. Allow to flash (paint goes dull and flat) between coats, then apply a light guide coat of a contrasting colour like black, and using 1500 grit wet&dry de-nib the primer - wet rubbing using plenty of water. Highs and lows in the paint surface will be exposed by the guide coat. If you intend to use a high-fill 2-pak primer to enable you to get the absolute best finish, ultrasmooth and mirror-flat, make sure you use a proper respirator.
Hint: practice on something that doesn't matter first ......
June 88 535iA, 173,000 km; Sep 00 735i 170,000 km