You can add something to acrylic paint so it levels better and won't dry too fast. Liquitex brand slo-dri and flow-aid. Both of these will help the paint spray better, level out and not dry before it hits the surface. The paint actually drying before it hits the model is what causes the gritty texture. That can be removed by sanding with a 6000-8000 grit sanding pad/paper. However, make sure the paint is fully dry (24 hrs +). Both these products are at any art store or Hobby Lobby. For more extensive tips and ideas for acryl painting, go to the tools and tips section of the website below. Then go to "painting".
Andy