On the premise that a union has 2 paralell faces..and is not unlike but similarly found on older radiators, rather than the nut+olive connection that prevails nowadays. I reiterate that a smear of compound is effacacious on the face of the union. It should be diluted with a Plant based oil rather than an animal fat or crude oil derivative, because the product from British Steam Specialities, known as Boss White, is Linn seed based. Try putting Castor oil in a racing engine that`s been run on mineral oil without at least a thorough flushing or even a rebuild. 
