Primer generally contains a rust inhibiting element. It fills pores in the bare metal and protects bare ferrous metal surfaces. It is manufactured from (in paint terms) a fairly coarse material and cannot be prepared to the same standard as undercoat.
Undercoat however is manufactured from smooth fine material which gives a smooth finish. This product also has excellent high building qualities. It does not however protect bare metal. It should only be used if there are no bare metal areas present on the surface to be painted. If the surface is going to be used in a damp environment (such as on boats) then the surface should be prepared, primed and undercoated before applying the top coat to obtain the best possible result from the Coach Enamel.