Andrew,
If you want to use resin then put that on first, then the primer and then the paint top coat. No need for PVA. However with a balsa hull you might be better off using EZe Kote acrylic resin as it will sink into the wood and is water based. It also dries quickly so you can put several coats on in a short time – doesn't smell either! If you want to strengthen the hull surface against knocks then use GRP or modelling tissue with the resin and rub it down to a smooth finish before painting.
Colin