I used Brightsides gray primer on the hull of a Mill Creek three years ago, before putting on some Brightsides paint. It was mid summer, beautiful weatherwise, and the primer dried quickly. Within the day it was completely dry to the touch. I waited until the next day to sand, it sanded just as it should (a fine powder). I'm no expert on the stuff, but I can think of two possibilities. One is that you might need to give it a bit longer to dry, given the time of year (cold or humidity, or both). The other is that you have a bit of blush on the surface of the hull interfering with the primer.