Spray cans put on a pretty thick coat of paint. Pre-shading tends to get pretty well covered up with rattle can paint.
If you have not used a can before, don't start right off painting the model with it. Find some scrap plastic and use the spray can on that until you get the hang of it. Since you cannot adjust anything, distance from nozzle to surface, and the speed of movement of the can, are important.
Also, be careful on a large model. The cans cool down due to Joule-Thompson cooling, and that drops the pressure after awhile, and that reduction in pressure can screw up your spraying. If the can gets cold, stop.