Absolutely nothing, it is great Paint IMHO. I just always seem to get reprimanded when I dare to suggest guys paint there rides with Rustoleum (Tremclad in Canada).
I really think I may be painting my 47 Chrysler coupe with Tremclad mixed with automotive acrylic enamel reducer, and DOI wet look hardener.
I have sprayed this mix already on inner fenders, test pieces, etc, with great success.
My car is rough to begin with, never be anything more than a nice driver, so it may get the Rustoleum paint job, this spring.
Here are 2 pics of my car, this car was stripped, metal work done, some of the filler, it was then sprayed with rusty metal primer cut with medium speed enamel reducer, laugh if you want, but it's great base primer, next is filler work, primer surfacer, blocking,paint. The trunk is done in 2 k urethane high build primer, right over the rusty metal primer, with no isssues.........Fred