I'm looking to stain my outdoor yard fencing. The white cedar fencing is about 12 yrs old, in good shape, and is now a light grey color. What would be the most efficient way to stain the fence? I have about 350 feet of fencing for the project.
Focusing on efficiency, I would pressure wash and then apply whatever finish you choose with an airless sprayer.
Per @graphus I would start by pressure washing the fence, since this may improve the appearance significantly, and a thorough cleaning should be done prior to any application of finish. Pressure washing will be faster by far than any manual process. You will, however, have to wait for the wood to dry out before re-finishing it.
For applying finish efficiently (in terms of time) nothing will beat an airless sprayer. They move material out much faster than HVLP or conventional air spray guns. Airless sprayers can also spray higher viscosity coatings like latex paint, which HVLP really can't do. The clean-up and maintenance is a little annoying, and you end up wasting the material in the hose, but it is far faster than brushing or rolling, particularly in a situation like this where the surface isn't flat.
You will also have to consider overspray and "bounce-back", but since this is an outdoor situation the masking required should probably be minimal. If the fence is near pavement, trees, buildings, or anything else you don't want painted then it will need to be masked off appropriately.
Also note that you will need a sprayer tip that is appropriate for your material. Most quality coatings will include information about tip size. The tips are consumable and will need to be replaced every 10-20 gallons.