I think GTSDAVE's advice on surface prep is sound, but I would be hesitant to thin with laquer thinner simply because it's not the recommended thinner from the manufacturer. Gotta defer to the Rustoleum chemists IMHO.
I'm convinced at this point that the troubles folks are having are not a "hardness" problem, rather an adherence problem. My dilution ratio was significantly higher than the average (~45%), so you would think that it would take significantly longer for my paint to dry, yet it's hard as stone after only one week - unscratchable. I might be wrong here, but I think those having trouble started with a primer base. Did the primer coat folks sand down the primer coat before starting, or just wipe down with cleaner before starting? Maybe a primer coat alone isn't rough enough for the paint grip/adhere? Bare/clean metal painters, are you having problems? I know with products like POR15, a metal prep/etcher is needed, something to "rough up" the metal enough to give the POR/paint something to grab onto. Hope this helps.