I had the bottom of my 59 "Rhino-lined" along with the interior floors and smuggler's box. (Epoxy primed the frame and just but a good heavy coat of black paint on it.)
Not so much for rust prevention----but for sound deadening.
Kept all drain holes open, cause not only does the moisture need to get out, air needs to get in to help dry everything out when it can.
Again--the car was apart when this was done.
A few years ago, one of the "Big 3" used to dip the pickup/truck frames in some sort of wax.
Be interesting to try to duplicate that--I think that stuff stayed on for a long time.
However----a good cleaning and epoxy coating on the frame is probably the best route