Spray coating can be made to work. We do it in our lab commonly with our ultrasonic atomizaton process. The initial tests showed there were challenges in adhesion, but that is not limited to the spray technology and can be overcome using the correct carriers and surface preparation. Recent innovations have shown impressive leaps in homogeneous, uniform coatings when electrostatic fields are applied. We do all this here.
1. The requested thickness of graphene coating over the substrate
2. Type of solvent that you want to use for the coating
3. What are the type of substrates that you are going to use etc.
I would recommend Robb Engle's answer.
Also, commonly, for a very thin graphene coating, I would suggest the spray coating in the ethanol medium at around 40 degree Celsius.(think about the stability of your substrate).