ALWAYS avoid a single, heavy coat !
This can, and probably will, lead to "pooling", runs, sags and drips, no matter how the paint or varnish is applied.
Whether using a brush or airbrush, apply one, even coat - missed or "thin" areas can be covered before the first coat is dry - and then build-up the coats as required for the desired finish, once previous coats have dried.
Also, find the optimum distance, from spray can or airbrush nozzle, from which to spray for the medium being used, by test and practice - too close can cause bubbling and an uneven, "runny" surface, and too far can cause a grainy, rough finish, or a thin, patchy,. uneven finish.
In general terms, spray cans are jus about acceptable for covering large, even areas, where a constant angle and distance from the surface can be maintained, but are not really suitable for anything else. They also lack good control over pressure and spray pattern an, in the long term, work out as expensive, compared to "normal" paint used with a compressor / airbrush or paint brush.