For the curative (it's not a universal hardener) to have any effect, it has to find compatible sites on the other molecules with which to react to form the chains and branches of a polymeric network. The common functional group on the resin side is the hydroxyl group, -OH. These react with the isocyanate -NCO sites on the curative to form urethane groups linking the moleculesinto a polymeric network. A&B, hook and chain. Not being an egghead, just the fundamentals. If you have reactive sites, something can happen. If not.. nada. Castor oil is a triol - three reactive OH groups per molecule. Typical elastomers have a bit over 2 on average, resulting in lots of chains but not so much branching = mobility and flexibility. More branching = more rigid. Bla bla.
Point being it is that simple.. A reacts with B to form C, which is really ABABABABAB linked by the common urethane group (and in 3D). So if the enamel happens to have some reactive sites on the molecule, it could crosslink a bit and increase chemical resistance and toughness. Key word = "if". I've seen reference to this trick in auto paint discussions as well. I'm interested to find out if there is any legitimacy. I've seen enough completely bogus "hot tips" to be skeptical until proven.
Nitrocellulose lacquers will crosslink btw - there are reactive -OH sites on the NC backbone.
Yes in general you will find that your automotive two part PU finishes are tough and quite chemical resistant.
Last edited by MJD; 12-28-2014 at 10:37 AM.