i use hair spray on all my airplanes....
1 i use light weight spackling compound, airspray helps monocoat stick to the spackle
2 hair spray the cheap stuff acts like balsarite
3 helps monocoat adhere
4 seems to help the wrinkles come out to a point.
here's what i do, after sanding lightly spray the aircraft with hair spray, light sand, spray heavier and light sand again, apply monocoat as usual