Yes - Sound deadner as DDaddy indicated. Siumilar was "Rubberized undercoating" used to be sold in aerosol cans at the parts stores. Some of the restoration oriented owners have discssion about the best replica.
Like others, I never liked the fact if there was any crack, or it started to flake, moisture would get trapped against the metal.
My car only had it in the wheelwells, so I left it. The rest I painted with marine epoxy. Laying on the the floor - car on jackstands. Lots of fun - not.
But no complaints it was a desert car without the general undercoat option. Could have been a lot worse!
Rustoleum changed the formulation of their rusty metal primer. Its too soon for me to comment on whether it is as good as it was. Yes - like most primers it has high permeability and needs topcoating to protect the metal against moisture.
Corroless is a different story. When dry it is highly moisture resistant. It uses several different technologies than other rusty metal primers. Its a British product that as far as I know, currently has no distributors or retailers in the USA. If you find otherwise, please let me know. They might or might not ship it direct. Prob the easiest way is to find a UK source who ships.
IIRC the one paint that can not be used over Corroless is Hammerite. Hammerite is styrenated something or another. Its licensed for manufacture here in US by Masterchem so readily available.
Dave answered that.
For nooks, crannies and other places that can't be properly prepped and painted there are a number of non-curing waxy products. Waxoyl is one I've used - also a UK item but available here. Lets face it face, the Brits are rust experts. View attachment 1715421616 View attachment 1715421617 View attachment 1715421618
