keeping dust out of paint

The dust most likely isn't in the paint itself, at least not inside the can, it is either on the model surface before you spray it or settles on the surface while the paint is drying. To resolve the former clean the surface well before spraying. I prefer alcohol and an air compressor, others use a tack cloth or something that will pick up dust. For the second problem, take a box large enough to cover the parts being painted, cut large holes in the sides and top, and cover the holes with coffee filters. They will allow air to circulate but will keep out dust and dirt.

The main thing I suppose is to try an paint in a relatively dust free area. Although it is important to have good ventilation, don't paint in a wind storm, whether natural or artificial. If you are spraying a gloss paint that takes a while to dry, you can put a dust-free cover over the model while it dries so that dust doesn't settle on it. After the paint has dried, if there are things that have settled onto the paint, sometimes you can remove them by brushing over them with a fairly stiff brush.

Andy