How much primer do I need to use to be able to paint a grey color over a bright blue wall?

Hello there, thank you for your question. Each gallon of our All-Purpose (Kilz 2) Primer covers about 300-400 Sq. Ft. We'd suggest applying one coat of primer and topcoating a test area, once the primer has dried, with your paint to ensure you are obtaining your desired coverage. Hope this info helps. Happy coating, and best of luck with your project! -Joanna