'Liquid' rubber is used in stabling, barns, livestock lorries etc. Its a bit like fibreglass - you have your solid materials and a resin. You mix them to a thick paste and you can lay it like you would with wet concrete. When it goes off, you get a solid rubber floor. Don't know if this was what you read about, but that's the only liquid rubber I know of
From the other side of the language barricade , the "liquid rubber" sounds like a black coating material used for anti-rust protection of the parts like floor, wings etc. which, even after hardening, is somewhat "rubbery"