How do i repair or stop the cracks in the bumper of the mid 70s laguna?

i've jest had the car painted early winter last year and in jan. i notice a crack in the nose when the summer got here there were more crack this car has been in the inside all winter the person that painted the said that theres no way to stop the cracks the car hasn't been in any accidents the car was painted with base coat clear coat with flex additive