I think they may have switched as I thought it was a polyurethane, then found it was acrylic lacquer. Acrylic lacquer is fuel proof, other lacquers are not. Acrylic enamel is not fuel proof, some other enamels may be. Epoxy and Polyurethane are fuel proof. Apparently some fish oil based enamels (Rusoleum) are fuel proof IF and only if you let them cure long enough.

I think Lustercote has a cheap thinner which has little or no plasticizer, which means it will easily crack, especially if too hot when drying. You can find a better acrylic lacquer at yuor auto parts store. Costs less too. Name is Plasticote Truck and Van, some other Plasticote brands may be acrylic lacquer, but some are enamals, read the back of the can to be sure.