It should not be considered part of the coating system. To put things in perspective – a coat of wood primer is applied at around 32 microns, while factory applied timber primers typically have film builds between 10 – 15 microns thick.
This means timber weatherboards need to be kept dry until installation – the “priming” does not weatherproof them.
Exposure to the elements increases the risk of swelling and primer breakdown – so keep your timber weatherboards off the ground, indoors in a well-ventilated area until installation. A secondary cover and groundsheet should be used if weatherboards are stored outdoors.