I can tell you what I know, it's not much.
While at a car show, I asked what paint they used to paint their engines. The answer was Centari Acrilic Enamal with hardener. The paint seems to last quite well on hot big block engines. Using hardener gives the look as though clear-coat was used. I've painted a few fenders, lawn mowers, truck cab, car frame with this Centari and I like it. It's not cheap unfortunately. O'Reilly main stores will mix paint and put it into spray cans for you. That seems to be good paint but it's not Enamel. It's hard paint and chips easy. They also cannot mix hardener with it before putting it into the spray can.
Plastic parts need a special primer and elastic paint that flexes with the plastic. I'd suggest going to a automotive paint supply and talking with them about what would work well on that air dam. I'm certain their going to say Acrylic Enamel but for primer, I'm not sure about. I've learned that Laquire paint is very hard, that's great for doors & trunk lids but not so good where sand & small rocks can hit the car (nose area) I can't recall the name of this really tough paint, it's too hazzardous to use outside commercial businesses. Brain damage & all that. It's mostly Enamels and Acrylic Enamels today. Water base paints? I haven't a clue.
My best advice is paint suppliers. Your going to need a special primer in order for any paint to stick to the flexable part. Can't use clear-coats because it doesn't adhear that well to paint. (ducking to avoid rocks & bottles)
EDIT: I'm sorry, my mind has been on paint & prep answering your question. I wanted to mention how nice your paint work was before the clearcoat cracked. I know how you must have felt when this happened and I'm sorry. Need to meantion humidity also has a part in how well paint sticks to parts. Last paint I used came from Atwoods farm supply. They have industrial Enamel. Van Sickle Super Premium. Either in mixable cans or spray cans. I thinned the mixable with mineral spirits or VM&P Naptha and used Glossy Acrylic Enamel hardener. They say the slower paint cures, that better. Mineral Spirits is really slow, give it three weeks to cure. VM&P is faster, both turned out great, so far...