I read the Add/Tech Sheet. It suggest that 1 coat should be enough to do a car...if it says it should be enough, apply at least two...if nothing else than for the protection that paint gives versus primer and how uniform the color will be after it has been applied. The price is right at $25.00 a Gallon for low gloss Black, perhaps buy 2 Gallons of paint reducer and hardner. By the reduction numbers 8 : 1 : 1, it sounds like a very common Acrylic Enamel reduction ratio.
This product may need to cure before the paint loses it gloss, so if you paint it and it seems to stay shiny, let it dry completely before you expect a flat finish, when the solvents evaporate, the color should die down.
Let us know how it turns out...sometimes durability isn't reflected by the price and at $25.00 a Gallon, it could be a tight budget answer.