Nearing the end of restoring my father's 64.5 convertible now. He bought this car in '69 from a Parts Manager at a Ford dealership. Used as a daily driver until '71, it wasn't until the late '70s until he started his 'restoration'. New Jersey tosses plenty of salt around and there were the usual panels to get replaced. After the repaint, he invested in a full Tuff-Kote rustproofing, and this (plus garaging) is what has kept the car solid.
We did a full strip down and soda blast on the vert before repaint, but I purposefully left the underside alone.

Tuff Kote in place is still semi-solid, waxy brown/black. Door/trunk/other inner lower cavities have coating there like the day it was done.
But the lower firewall to about back to the trans tailshaft has lost the coat- oils from 110k of leaks stripped it over time.

I have drilled the internet for a compatible product that I can use to re-coat. (Tuff Kote was bought out by Zeibart in the early '80's).
Rubberized products probably are not compatible and I fear trapping moisture.
Fluid Film does not seem to be solid enough and I don't feel like recoating every 3 years
I used POR-15 on my '55 F-100 frame but compatibility is doubtful.

Welcome any ideas, even recipes of things to combine.

 

Have you checked with a local body shop supplier and even the body shops on what they use on new cars & trucks.
Might find something that is close that will work now.
Dave ----

 

I have checked out this product and it looks perfect for what I'm looking for. It will be a few months before I attack this messy job though and I will circle back to confirm.
Kudos to "Woodchuck" on the Vintage Mustang Forum!

I think the closest you will come to it is.... Waxoyl Black Hardwax Underbody 5 Litre (1.32 Gal) Tin RNW5005 | Rovers North - Land Rover Parts and Accessories Since 1979