Stop in for a cup of coffee

Ddaddy said
Nobody really uses undercoating much anymore as it caused more rust than it prevented. Back in the vintage car era, they used to do a lot of spraying of the undercarriage with oil as a seasonal protectant.

These days, the preferred spray for the undercarriage is “Fluid Film” lanolin spray. Works great!

https://www.lowes.com/pd/Fluid-Film-1-oz-Lanolin-Based-Petroleum-Oil/50335157?cm_mmc=shp-_-c-_-prd-_-tol-_-google-_-lia-_-217-_-tools-lubricants-_-50335157-_-0&store_code=622&cm_mmc=src-_-c-_-prd-_-tol-_-google-_-tools-_-MHLIA_TOL_Tools_High Priority-_--_-0-_-0&gclid=EAIaIQobChMIlvOEsuDg5QIVDvbjBx0jYwjOEAQYASABEgLkovD_BwE&gclsrc=aw.ds

I have been all over the net now to look. I have been used to the Valvoline Tectyl, which had a cavity wax, and an undercoast that was paintable. Now they have only the undercoat, Tectyl 506. I have seen the Fluid Film you mention, which I assume would be excellent for cavities, bottom of the doors for example.

My 1965 Dodge Dart has had some kind of undercoating under the wheel wells, like Tectyl. But, could be any brand. It is not like it smells anymore, so it is difficult to say what it is. Right above the front wheels the sand and dust has worn through the undercoating, and it is a tiny layer of rust. Something like a thin dust of rust. The undercoading on the sides of that is perfectly fine still.
I don't know when this was done, if Dodge did it when the car was new, but it has obviously worked. For some reason my car is what I will call rustfree, this little dust has not rusted into the sheet metal.

I want to put something on it, Rust Oleum have a primer that is supposed to work well on rusted surfaces, and then there is another reddish primer called Corroless that I have used in the past with a very good result.
However, I need something on top of that. Primer alone is not going to hold up too well.

Bill