A method of coating a substrate with a washcoat, comprising the steps of: engaging the substrate with a headset of a substrate coating apparatus so as to locate an upper surface of the substrate below a washcoat showerhead of the substrate coating apparatus; conveying washcoat from a source of the washcoat towards the washcoat showerhead; using a valve assembly to control flow of the washcoat from the source into an interior of the washcoat showerhead; discharging the washcoat out of the washcoat showerhead onto the upper surface of the substrate; and drawing the washcoat through the substrate by applying a suction force to a lower surface of the substrate; wherein: the valve assembly comprises an outlet valve (23, 24) movable between a closed state and an open state to control flow of washcoat into the interior of the washcoat showerhead, the method being characterised in that the valve assembly creates a pressure drop within an interior of the washcoat showerhead when the outlet valve (23, 24) moves from its open state to its closed state; the outlet valve (23, 24) comprises a valve stem that reciprocates relative to a valve seat; the valve assembly creates a pressure drop within a valve chamber of the valve assembly when the outlet valve (23,24) moves from the open state to the closed state; and the valve stem functions as a piston within the valve chamber and creates a pressure drop within the valve chamber when moving from the open state to the closed state. engaging the substrate with a headset of a substrate coating apparatus so as to locate an upper surface of the substrate below a washcoat showerhead of the substrate coating apparatus; conveying washcoat from a source of the washcoat towards the washcoat showerhead; using a valve assembly to control flow of the washcoat from the source into an interior of the washcoat showerhead; discharging the washcoat out of the washcoat showerhead onto the upper surface of the substrate; and drawing the washcoat through the substrate by applying a suction force to a lower surface of the substrate; wherein: the valve assembly comprises an outlet valve (23, 24) movable between a closed state and an open state to control flow of washcoat into the interior of the washcoat showerhead, the method being characterised in that the valve assembly creates a pressure drop within an interior of the washcoat showerhead when the outlet valve (23, 24) moves from its open state to its closed state; the outlet valve (23, 24) comprises a valve stem that reciprocates relative to a valve seat; the valve assembly creates a pressure drop within a valve chamber of the valve assembly when the outlet valve (23,24) moves from the open state to the closed state; and the valve stem functions as a piston within the valve chamber and creates a pressure drop within the valve chamber when moving from the open state to the closed state. the valve assembly comprises an outlet valve (23, 24) movable between a closed state and an open state to control flow of washcoat into the interior of the washcoat showerhead, the method being characterised in that the valve assembly creates a pressure drop within an interior of the washcoat showerhead when the outlet valve (23, 24) moves from its open state to its closed state; the outlet valve (23, 24) comprises a valve stem that reciprocates relative to a valve seat; the valve assembly creates a pressure drop within a valve chamber of the valve assembly when the outlet valve (23,24) moves from the open state to the closed state; and the valve stem functions as a piston within the valve chamber and creates a pressure drop within the valve chamber when moving from the open state to the closed state.