Primers are often missed out by even reputably good quality powder coating companies.
An epoxy primer presents an impermeable barrier. It is applied under a polyester top coat which is not fully water proof.
Unfortunately, Epoxy is not UV stable and needs to be protected from sunlight; otherwise it will oxidise. This oxidisation shows as a white deposit sometimes referred to as chalking.
The epoxy primer often costs more than industrial polyesters. It is also an expensive process to apply because the component is powder coated twice.