Peckinpah, It would be wise to experiment with the thicker coats of paint on an old fender, mail box or something. I'm going to guess that thicker paint will mean trouble getting the paint to cure in a reasonable time frame. If the paint is not sufficiently dry, it will be impossible to sand and polish it. (It will gum up the sand paper) I've seen thick coats of paint take many months to cure.

If a hardener is mixed into the paint, the thicker coats MAY work. (Hardener is Dangerous, so proper safety gear is required) The second option would be to bake the paint. (No idea on the required temperature, time needed, etc,)

One final thought, read the find print on the paint can. A lot of paint manfacturers recommend paining at temperatures below 90 degrees. You should also be aware of metal temperature. If the car has been out in the hot sun it needs time to cool before applying paint.