A printing apparatus, comprising: a gas source operable to provide oxygen and to provide a non-reactive gas for an inerting gas; a controller operable to control a level of oxygen and a level of said non-reactive gas to vary a composition of said inerting gas from said gas source; and a printer comprising: a sequential in-line printing assembly comprising: a base coat printhead; an inerting gas applicator; a curing region configured to provide illumination; and a top coat printhead; and a transport system for transporting a substrate through said sequential in-line printing assembly such that said substrate is sequentially treated with a base coat ink, an inerting gas atmosphere, curing illumination from said curing region, and a top coat ink; characterized in that said gas source is coupled in fluid communication with said inerting gas applicator, wherein said inerting gas is delivered to said sequential in-line printing assembly via said inerting gas applicator; and wherein said controller is configured to vary said level of said oxygen and said level of said non-reactive gas in said composition of said inerting gas, to controllably deliver said oxygen through said inerting gas applicator in a range that simultaneously provides in a given print job both sufficient spread of said top coat ink, and interlayer adhesion between said base coat ink and said top coat ink; and wherein at least one print attribute is changed by altering chemical levels in said inerting gas. a gas source operable to provide oxygen and to provide a non-reactive gas for an inerting gas; a controller operable to control a level of oxygen and a level of said non-reactive gas to vary a composition of said inerting gas from said gas source; and a printer comprising: a sequential in-line printing assembly comprising: a base coat printhead; an inerting gas applicator; a curing region configured to provide illumination; and a top coat printhead; and a transport system for transporting a substrate through said sequential in-line printing assembly such that said substrate is sequentially treated with a base coat ink, an inerting gas atmosphere, curing illumination from said curing region, and a top coat ink; characterized in that said gas source is coupled in fluid communication with said inerting gas applicator, wherein said inerting gas is delivered to said sequential in-line printing assembly via said inerting gas applicator; and wherein said controller is configured to vary said level of said oxygen and said level of said non-reactive gas in said composition of said inerting gas, to controllably deliver said oxygen through said inerting gas applicator in a range that simultaneously provides in a given print job both sufficient spread of said top coat ink, and interlayer adhesion between said base coat ink and said top coat ink; and wherein at least one print attribute is changed by altering chemical levels in said inerting gas. a base coat printhead; an inerting gas applicator; a curing region configured to provide illumination; and a top coat printhead; and a transport system for transporting a substrate through said sequential in-line printing assembly such that said substrate is sequentially treated with a base coat ink, an inerting gas atmosphere, curing illumination from said curing region, and a top coat ink; characterized in that said gas source is coupled in fluid communication with said inerting gas applicator, wherein said inerting gas is delivered to said sequential in-line printing assembly via said inerting gas applicator; and wherein said controller is configured to vary said level of said oxygen and said level of said non-reactive gas in said composition of said inerting gas, to controllably deliver said oxygen through said inerting gas applicator in a range that simultaneously provides in a given print job both sufficient spread of said top coat ink, and interlayer adhesion between said base coat ink and said top coat ink; and wherein at least one print attribute is changed by altering chemical levels in said inerting gas.