Hi all. Just starting, slowly, on the dismantling of my '82. Finally got all the pecan shells vacuumed up, then went at the interior floors with chisel to remove the sound-deadening stuff. I've found pretty typical rust but, maybe, not as much as I feared. What I have found is lovely and smooth black painted finish under portions of the sound-deadening. Sure looks good but...not sure if it is good. If the finish is nice and smooth looking does that mean there is NOT rust underneath and I ought to leave it alone? Or ought I just scrape into it and see what is under the black paint?

That shiny black paint might be a product known as POR-15, a very popular product with collector car enthusiasts!

I'd be inclined to scrape a screwdriver blade thru one location to satisfy my curiosity.

I had black paint on my undercarriage when i scraped under the rustproofing but on the interior i had rust underneath the sound deadening membrane. Usually you will see hint of the original color of the car. It wont hurt to sand down an area to see what's underneath the paint. If you apply the por15 product to your floor be sure to degrease and to etch the area first, those two steps are just as important as the por15 treatment itself. If you do it right you will never need to worry about your floor for the remaining life of the car.

I have just stripped my air filter back to bare metal (& a bit of rust) towards applying POR15 & an enamel topcoat.
Is the POR15 ok for use in the engine bay where things get significantly hotter than the chassis $ floor that it is used for most often?

You can use it the engine bay including the block . A top coat is required.

you can not beat a good epoxy primer .... just my 2 cents

and I agree with scratching little spot to satisfy your curiosity .... it was not hard to see where there was rust ... clean smooth shiny black metal would get checked off as good in my book

If you use POR15 make sure you read the directions concerning adding a top coat, this must be done before the POR15 is cured or it will NOT bond......trust me I know. It is a great product but you must follow the directions!!

As with any paint, the secret to longevity is prep. The paint can be the finest in the world but will still flake off with bad prep. Pors is a polyester paint. My experience with polyester is 15 years in the fiberglass business. Standard boat resin, the kind of stuff you find in bondo has a high end temperature tolerance of around 110 degrees F. To get higher temperature tolerance you need an isophthalic resin. Tooling gelcoat is made from this stuff. Has temperature tolerance in the 400+ range. I don't know what Pors is made with, but my guess is its not isophthalic. Epoxy, on the other hand, has very high temperature tolerance. I think in the 500 degree range if i recall correctly.
Both Polyester and epoxy are a chemical cure and are other extremely resistant to any solvents, even acetone. Also Polyester gets harder and harder as it ages. This is why there is a window for coating with a top coat. I used epoxy primer when doing my car. It spent a few years in the primer state working on various parts of the body. The entire time, my car was exposed to extreme heat (arizona summers) and lots of UV. The epoxy primer never deteriorated and remained sandable. Bottom line here is the same with polyester primers, dont expect a chemical bond with the top coat. The bond will be purely physical and required complete sanding of all common surfaces.