Some other options with "real automotive paint...

Restoration Shop Colors

They have single stage poly gallons with hardener for under $100.

as do these guys:

http://www.smartshoppersinc.com/kirker/acrylic-urethanes.html

I have used Kirker paint, in fact, shot the 300D in GM #81 10 years ago and it was fine.

Not sure I see the benefit of doing it the "rustoleum way", unless you are starting off with no compressor, no gun, no place to work. You can't save anything on the paint and I doubt the durability will be anything like you could get with a real auto urethane, even a cheap one.

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80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)