I also see you can get vapour oil patches to stick inside a gun cabinet – are they of any long term use?

I assume you are referring to those little packets of silica gel that are often packed with new guns, as well as with many different kinds of mechanical and electrical machinery.

They are intended to keep the air inside the case dry while the gun, or whatever, is in transit to the retailer – nothing more.

They certainly won’t absorb enough water to dry out a wet gun.

Those vapour oil patches for gun cabinets are quite efficient, as long as guns are put away dry.

If you do get condensation inside your cabinet (usually when a cold day follows a warm, damp spell), then they do prevent rust.