I sprayed every piece of metal on my jeep truck inside & out...
By the way that chassis black looks good sprayed on top of the por-15 like someone else said 1 QT goes a long way....3 qt's did every thing on my 4x4 right down to the nuts & bolts! well i may have gotten into a 4th. Do wear a mask & gloves..i've hand black hands for days when i didn't wear gloves.. the stuff is just that thin you can't help but get some on ya!
Oh and i been using the por-15 for around 20 years now! & they have came out with some great stuff since then...

Best of luck...you'll be hooked once you use it.