I'd use latex house paint and brush it on, then a roller for consistent texture final finish.
I've done many this way and they look nice.
Not familiar with HFT airless unit, I've painted a house with metal siding and a good airless unit and it looked pretty nice. You do have to thin the paint a fair amount with water to get it spraying right, so runs are a worry.
Car paints with a gravity gun seems like too much trouble for what you want.