You will need enough rust converter / metal primer to cover the rusted area, and enough plastic film sheet to cover the treated area.
First ensure the rust is only surface rust and the tank is sufficiently strong enough to hold the weight of the water when it completely full. If in doubt, seek some expert advice on the structure of the tank before fitting the liner and filling it with water.
Using a wire brush, remove the loose surface rust from the inside of the tank. Treat the area with “rust converter/metal primer” available from your local hardware supply. Follow the instructions for applying the converter/primer, paying particular attention to working within a confined space.
Allow the converter / primer to completely dry and then install a protective sheet between the tank surface and the liner. You can use a Painters plastic drop sheet or builders film for this. If you are fastening the tank fixing extrusion at the top of the tank, you can secure the plastic sheet under the extrusion as you drill and rivet it in place. Add a layer to the floor of the tank if it has been treated with the paint primer and then using duct Tape, secure it in place before fitting the liner so it doesn’t move.
The plastic film sheet will act as a protective barrier between the rust converter/metal primer and the liner.