How should enamels be thinned

thin them to a milk consistency.. about 2:! paint to thinner... then play with the mixture until it sprays good...

Three words: Practice. practice, practice! Mixing to the consistency of milk is the most quoted rule, but I find making that judgement is somewhat difficult, and simply takes experience to develop. Before shooting on your model, be sure to test spray on some scrap plastic or other non-porous material: better to discover a problem on scrap than while spraying your model.

As far as adverse effects go. If you have "clumps" in the paint you get spurts through the airbrush. Don't ask me how I know Stir the paint well in the bottle. You can filter it before you put in your AB but that's alot of work. Add some thinner till it's a milk consistency like Tho9900 said. Then stir again in the color cup. I use one of those cheap nylon paintbrushes. You can use a dropper also. Those things are indespensible to have for working with paint in an AB.

I mix my paints in washed "Gourmet" pet food trays. Properly thinned paint will run rapidly down the vertical wall of the container but the streak will have good colour density. I find this more helpful than trying to get that "milk" consistency. With enamels I just use mineral turpentine as a thinner. Please make sure you wear a respirator when spraying as the vapour is definitely harmful to your existence! Well stirred paint is also the go.