Hairline cracks in paint

[Darth Lars]

I am trying to paint a helmet, and I keep getting hairline cracks in the paint appearing. It drives me mad!!
I sand it down, prime it, sand it some more, wait a day (40% humidity) and then I paint it. And then there are new cracks!
What am I doing wrong?

I have been using model master putty in some areas on the helmet, to repair some dents here and to smooth out a surface where I have resculpt. Could that have an effect?

 

[Dymerski]

I am trying to paint a helmet, and I keep getting hairline cracks in the paint appearing. It drives me mad!!
I sand it down, prime it, sand it some more, wait a day (40% humidity) and then I paint it. And then there are new cracks!
What am I doing wrong?

I have been using model master putty in some areas on the helmet, to repair some dents here and to smooth out a surface where I have resculpt. Could that have an effect?

Was it washed to remove mold release then primed?

 

[Darth Lars]

No. The helmet started out as fiberglass-reinforced styrene. It was primed. Some parts were already painted white - same brand and color as I am using now.
It is some acetone-based paint from the hardware store. I am not sure, but I think it is acrylic.

 

[Darth Detroit]

Sounds like there may not be enough flex in the paint.