Salt's mixed into the paint, among other things. Makes it harder for a demon to shift the stuff enough to get a hold.

[Constantine gestures for Tony to approach the circle as well, stopping him short of actually crossing any of the symbols.]

They're a design of my own, a mixture of different symbols from different origins for binding and holding. Not much can get through this easily.

And as for the other ingredients...

[Constantine returns to his bag and produces a sealed jar filled with a thick, vile looking sludge. It has a distinctly deep red colour to it that suggests that maybe Tony wasn't entirely wrong with wondering if blood would be involved.]

Here's one I prepared earlier.