It can be used on old or new metal and may be painted straight onto rust. It binds to the surface and reacts chemically with any rust that it comes into contact with, neutralising the rust and preventing further corrosion. The second coat acts as a barrier to give additional protection against future rusting. Minimal surface preparation is necessary - removal of loose and flaky material - and it can be used on damp or even wet surfaces. No over-coating is necessary, but it acts as an excellent base for most standard paints if over-coating is required.