Painting a rusty galvanised metal water tank requires the same process as other painting projects. You must clean the surface, apply primer and then paint as you see fit. However, the material and shape of the tank make this process trickier. Also, galvanised metal isn't supposed to rust, so if your galvanised metal water tank is rusty, you'll need to check its integrity. Round tanks respond better to spray painting than they do to brush or roller painting.
Hold the screwdriver in your non-dominant hand and press the tip against several large patches of rust. If the screwdriver pokes through the surface of the tank as you press on it with your non-dominant hand, the tank is too rusted to paint. You should repair or replace it before moving forward.
Spread dust sheets around the base of the water tank. If the tank is small enough to lift, pick it up and place it on the centre of a dust sheet.
Scrub off all rust using steel wool and a wire brush. If at any point you scrub all the way through the wall of the tank, discontinue the project. Your tank is too rusted to paint.
Fill the reservoir of your paint sprayer with a primer rated to adhere to galvanised metal.
Apply a coat of primer to the surface of your water tank. Use long, even strokes and move slowly to ensure a complete coat.
Allow the primer coat to dry. While you're waiting, empty and clean the paint reservoir, then refill with paint.
Apply a coat of paint using the same technique you used to apply the primer. Allow the paint to dry, then add a second coat if necessary.
Outdoors is the best place to use a paint sprayer. If you must complete this project indoors, wear an air filter mask and open as many nearby windows as possible.