I think the main thing is getting the surface properly scuffed....and being patient on letting the enamel dry all the way thru. Enamels are a very soft paint with long, long cure times. If a hardner is added, it accelerates this process and thus makes a more brittle paint--and in my mind, will hurt the roller method as you need time for it to level. Mineral Spirits that are in the paint do not evaporate very quickly, say as acetone, lacquer, or keytone. For an enamel.....I'd say a month would be a good time period to say "it's finished" and to do any scratch/bang/beat testing. it will never be as hard as an enamel with hardner, but shouldn't be as prone to crack at seam joints, etc.