Back priming is the process of priming the “back side” of material to be hung. Usually it is exterior siding. Sometimes it is interior siding or another product. The idea behind this is to add an extra layer of “security” in case water migrates to the back side of the material. It also minimizes warping of the siding.
We have done a fare amount of it but I’m neutral as far as my opinion is on the necessity of doing so. Back priming seems to be more of something a particular architect gets locked into and then never deviates from. For example many years ago we did two Pier One stores. At that time the facade was a stained lap siding with painted black brackets connecting the various posts and beams. The two stores were in different states and had different general contractors. One store they insisted the siding must all be back primed. The other store it was not a requirement.
It is a real pain in the butt to prime a piece of lap siding when you know the other side is going to be stained and so no primer can touch or run onto the other side. We got it done.
Fast forward many years and at about the 15 year mark the two buildings looked the same. A couple years later one building was torn down and the other was turned into a health clinic.
There are probably valid reasons to back prime in certain circumstances however in the real world I see little difference as long as the exposed side receives the proper prep and paint.