Whole PF is sanded back right at the start... removes any loose and flaking paint and also gives the new paint something to bite into. Any chipped paint should have its edges sanded to smooth if possible.
Vinyl i use is STENCIL MASK by AVERY... there are a few stencil vinyls out there and ive used most of them but have recently come back to the Avery brand as its a translucent blue and so you can see through it. For text areas etc i cover the mask with clear application tape, so i can align correctly. Mostly though i use a low tack paper app tape.
Often these days i dont stencil as much, i can balance my airbrush to give very thin lines and i can pretty much go to an edge free hand with it.
Yes.. take lots of pictures before you pull a playfield down... even then you end up with bags of parts in front of you while you scratch your head!
Best thing to do is just start somewhere... if you know this part definately goes in that place then put it there... now your pile of bits is a little smaller and its getting easier. All posts i normally place loose around the PF before screwing them down... allows you to determine where the different heights go... which ones need to go through a plastic etc
