It's just like some self levelling paints like some primers for example they have are quite heavy and take a while to set giving them time to level out any imperfections , capilliary action an surface tension stuff. If you have a White zed and it gets scratched it won't hide the white pigment underneath it just seals itself that it the clearcoat on top. So in theory you will never have a rusty car but any chips will still be visible if the pigment underneath has been chipped away.