Mr.Whitey

I just painted the brace that attaches the two SLP SFC together in the middle with gloss black Rustoleum. I put like 5 thin coats on and let it dry for two days. Today I just wanted to be sure it was dry so I barely touched it with my finger nail and to my horror it left an indentation in the paint. Besides getting into something like POR-15 is there any paint that will be harder than what I've just used? It looks great but there's no way it will survive under the car like it is now.
I even used a hair dryer to aid the drying. Did this for some unknown reason prevent the paint from hardening properly?