How did you rustproof your bottom?

Hi Guys/Gals: I know this topic has been addressed before, but please offer your opinion. My very stock 1974 Duster has some surface rust on the bottom. I plan on keeping it forever, and I don't want the rust to become a problem to where I need to replace the trunk floor, interior floor, quarter panel extensions, etc. I'm thinking that there must be some simple one application treatment that I can use to coat the underside of my car so that I don't have to worry about rusting it out when I drive in the rain. Perhaps something like a POR-15 product that you just brush on, and it converts whatever rust that is there into a rust free impenetrable surface. I don't want to wire wheel down the bottom of the car and make it look new. There's gotta be some easy to apply one coat product that you can use by simply giving the bottom a good wash, then brushing it on. My metal is solid - for now. I want to keep it that way. Any and all ideas are welcome, but hopefully someone can recommend a simple one step process. Thanks!