Apply one to two coats of a clear wood sealant. Use a paintbrush to paint the clear sealant directly onto the pressure treated wood that you wish to gray. When you use a clear sealant the color of the wood will fade faster than if you use a sealant that contains a stain or pigment.
Use a clear wood sealant that does not contain ultraviolet stabilizers. Ultraviolet stabilizers are ingredients in sealants that help protect the wood from the sun's penetrating rays, which are known to speed up the graying and weathering process of the wood. If your goal is to make the pressure treated wood become grayer, find sealants that are not made with ultraviolet stabilizers.
Avoid power washing the wood to restore its natural or artificial color. Power washing is a cleaning technique intended to bring back some of the brightness of the wood's color. Expedite the graying of pressure treated wood by taking care of the wood and washing it with a rag that is soaked in water.
Leave the wood out in the sunlight. Sun exposure will speed up the graying process by naturally stripping the wood of its color and giving the wood a more worn and weathered appearance. When the color fades to the appropriate gray appearance you are after you can bring the wood inside or in the shade to avoid any damage to the actual wood.