That cracking of the over coats is most likely due to solvent retention. Applying coats over a coat that has been left too long or after sanding, without leaving time for the solvents to bleed out, causes the crazing. I always leave it at least a day if I've sanded a coat or if I haven't applied the further coats within the specified time. Instructions on the can usually say 'overcoat within a certain time or wait a certain time before overcoating'. If you don't do it within a set time, you must wait a set time before doing it.