For this reason, I carry a spare set of through-hull screens that I swap out every three to four months. I spray the spares with Pettit Prop Coat Barnacle Barrier 1792, a gray spray paint that comes in 16-ounce cans. I also replace the stock brass Phillips machine screws that hold the screens in place with stainless hex bolts. Hex heads make it easier to remove growth-covered bolts underwater. I use a fresh set of bolts, with a little Tef-Gel (tefgel.com) on the threads to help them come out easier later on, when swapping out the screens. After removing the old screens and bolts, soak them in Barnacle Buster overnight, then clean with a wire wheel. Repaint the screens a day or two prior to swapping them out to make sure the paint is at its freshest.