Trunk Floor Rust Protection

I've just finished replacing the two end portions of my rear frame rails and trunk floor. The frame rails are treated with weld thru primer, but the trunk is bare metal. I'm going to prime and paint the top side with gray speckle paint to the replicate factory finish. But what would you all suggest for the underside? I want to try to prevent rust from starting up again. Before I reinstall the gas tank, I would like to get the work done while everything is still exposed.

Thanks for your suggestions.

72 Rustecomy