Grainy Paint job

Too much psi. Not thin enough. Too hot of a thinner which promotes too fast of cure. Not adequately mixed paint.

Spraying to far away from the subject will cause the paint to start drying before it hits the surface - leaving you with scale roughcast.

Also:

• Insufficient pressure causing poor atomisation
• painting from too great a distance to the subject, allowing the paint droplets to dry in mid-air before they reach the surface being painted.
• using incorrect/incompatible thinner (I hope you're not using water to thin Tamiya acrylic paint)