Looking at the photo a few things could have happened here, either there's too much filler, too much paint, the paint never fully cured or wasn't mixed properly. All these would have been caught and prevented by an experienced painter. You like to brag about your sources so I'll let you in on a little secret, I've been in the automotive industry for 30+ years. Done properly the likelihood of it ever crackling is very slim.
Back to the point, rattle can paint (nothing wrong with it) has a little to no protective properties, if it did the auto industry like many others would adopt the formula.