taking apart/reassembling radiator: old RED radiator goop?

Hey all, got a question that I don't see anywhere.

Had to take some radiators apart to get downstairs out of a house, going to another house.



Now my question.



Years ago I worked with a ancient aged old real-true boilermaker pressure pipe fitter, my first experience of putting a radiator back together.

He had a can of RED colored horrible stinking goop that was brushed onto each push nipple.

I remember that the can actually had a picture of a Boiler and a Radiator on it, and on many radiators that I've taken apart I have seen hardened RED gunk on/around the push nipples.

I've also seen residue of it on the thread connections on old boilers systems



Obviously, it's an old product, but cannot be too old since I first saw it only about 10 years ago.



I figure, it must work, and must work well to last so long, just don't know if anyone here knows where/if I can get this stuff.



Otherwise does anyone have an alternate product/goop to seal around push nipples?





Thanks