Just googled and got this..
I've never actually used it when spraying mind
Benefits
Can be used directly over rusty surfaces.
Special anti-corrosive formulation guards against rust.
Ideal for exterior use - can also be used on interior iron and steel.
Provides a smooth, protective basecoat.
Can be overcoated with most conventional topcoats.
Preparation
Ensure all surfaces are clean, dry and free from dirt, grease and other contaminants.
Bare metal: Ensure any loose or flaking rust is removed by wire brushing. Sand down and wipe clean.
Previously painted surfaces: Remove any flaking or blistered paint/rust. Sand down and wipe clean.
Application
Stir well with a broad stirring stick to the bottom of the can.
Apply 2 coats using a brush suitable for solvent-based paints. Sand down lightly and wipe clean between coats.
Drying time
Touch dry in 2 hours. Overcoat in 6 hours. Drying times will be extended at low temperatures.
Hints
Not suitable for galvanised or non-ferrous metal, eg. aluminium, copper and brass. Use International Special Metals Primer instead.