One more tip to avoid the dreaded “orange skin” effect:
Make sure the hull is perfectly dry before you start painting. A good and fast way to dry the hull is having it upside down, optionally on saw horses. Then place a small, 25 watt lightbulb under it and leave it on for a few weeks. The heat produced by the lightbulb is just enough to dry the hull. And don’t worry about isolating, hot air rises up.
Don’t think this is a woody issue only, while a woody may be successfully painted with some moisture left in the wood, a GPR boat in particular must be absolutely dry! The more you sand the easier it gets to dry the boat. So, sand as soon as possible but postpone painting until just before the sailing season starts to allow maximum drying time (or power dry using a light bulb).
A light bulb also helps if you are painting in an unheated environment and the temperature is slightly below the optimum for the paint (usually 18C). A friend of mine successfully painted his boat just above 14C using the light bulb method.