I am working during the rainy season, and my chalk lines wash away if I don't spray them with polyurethane. Is there any type of chalk that will withstand bad weather, and eliminate this step?

IRWIN makes permanent chalk in either black or red. Because it won't rub, fade or wash away, black or red chalk is perfect for use on subfloors, concrete, metal, and roofs. There is no need to polyurethane. Just snap a line, and it's set. Red, black and yellow chalks are not removable, so keep them away from finish material.