So what other doors are there to open in the paint and undercarriage?
well, any catalyzed (hardener added) paint is going to be the most durable.. it's a better resin than an air-dried paint uses... question is whether ya go enamel, cheap, gal kits can be had from around $80 to $150... or urethanes, which usually run $150 to 400 for a gal kit.. a gallon of black Imron runs about $275, with catalyst..
I've recently changed up from running flattened black Imron to buying some Hotrod Flatz satin black catalyzed urethane.. kit was about $160 shipped..
hopefully it rivals Imron's durability...
once you get into automotive finishes, most of the major paint manufacturer's PPG, Dupont, Glasurit, Sickens, etc all have an single stage urethane line out... might be worth a trip to your local bodyshop supply store to see what they push...
now, all that being said, be aware the project can take on a whole other level of involvement when you get into spraying 2 part automotive finishes.. everything from compressor, to masking/overspray issues, ventilation, safety, etc...