Gloss or gas ghosting is still not well understood. Under the right conditions gases from solvents in the ink penetrate the surface of the paper and cause a change the gloss of the ink on the back side of the sheet resulting in a noticeable difference in contrast. The difference in contrast is the ghost and can be barely noticeable or distinct, similar to a dull spot coating. Sometimes recoating or varnishing the sheet will create an even gloss on the sheet, hiding the ghost. Adding ink drying stimulators and using heat from an IR dryer can cause the solvent gases to form more quickly and in some instances make the ghosting worse. Print the side with heaviest coverage first and allow it to dry for at least 24 hour before backing it up. Wind the delivery piles when the sheets can be handled is very important to disperse solvent gas buildup between the sheets. Also consider a high solids ink/low VOC formulation, these ink will have less solvent to cause gloss ghosting.