- You can spray over decal, but first few coats should be very light. (Actually, you should never spray heavily, whatever the brand is.)
- About 20 - 30 cm, depending on how quickly you move the can, and how thick you want to apply.
- Not necessary for a new can, but you might want to warm it up if it start losing pressure after using it for a while. The time depends on the temperature of the water.
(Personally, I never have to heat Tamiya or Gunze, but I had to warm up the Testor after using 2/3 or so.)