If its the floorpan only, I'd put it on like you said 70/30 for the first coat, and then full or near full strength for one or two more coats. I noticed in mine that there's just light oxidation after 28 years with only primer under the original underlay. Where it didn't have the underlay, the sandy grit does eventually pierce the primer and the rust wants to start to take root. Once you get the rust off, a few decent coats would probably keep the beast in good shape.
Since the tarred felt underlay didn't allow air to get to the lower floor pan, I'll use something like a fibrous rooftar over the paint, then put new underlay, then the carpets. We have a lot of snow and road salt, which does melt with the heater and soaks down, which may not be an issue for you. I figure the tighter i keep the air away, the less I'll have to watch the interior floor, and concentrate more of keeping it off the exterior of the floorpan with undercoating, etc.