I am getting into fluorescent ballast rewinding as a new hobby
In addition to my lighting collection.  As time moving on It getting harder and harder to get magnetic ballast for our flourescent lightng.  And tradition keeping old preheater
 Going and alive.   Even most members have stock up on
  Ballast.  And those who just enter collecting fluorescents
  Will have much harder time obtaning preheat or rapid start
 Ballast and chokes.  It one of reasons I am getting intrested
  In ballast rewinding.  Eventally I will be start selling rewrounded
 Ballast  with a exchange of ballast.

Nice, but this is bit of a mystery to me, how do they grade the wire and what the number of turns to do? 

 Once U remove all tar or pitch from the ballast.   I can take a common rapid start ballast remove the clips from the iron core and they come apart.  Now u got the nice I bar with the copper
Windings on.  Keep in mine the rapid start ballast is base off of switch start auto transformer ballast  with 2 or 3 extra windings
  To heat the cathodes until lamp get going.   And I just reverse
  Engineer the the auto transformer section 720 turns on the input coil and then u got secondary coil 1440 turns  with out put
 Of 200-210 volt.  Using 34 AWG high temprature magnet wire
   To rewind back on it I core iron.   When U get done rewinding this section then u have a choice from there  if u want over wind
 It if u want increase the OCV a bit or u can end it at that point
 Make a 40 watt preheat / switch start  ballast or wrap some small turns for the cathode heating for rapid start.   The switch start chokes can be harder to rewind do to typical E and I iron
 to get them apart to get to choke bobbin to unwind them. 
 EU chokes looked a lot harder to work with  with all welded metal base and it core  those seems and looked like it would 
 be a challage to even attenpt.  But I would be nice to reverse
 Engineer.