Cessna Cardinal 177 RG II v. Piper Arrow III (70s) v. Piper Arrow III (brand new)

Tell us more about that. What is it, that you do, on every other annual?

While the shop has all the covers off to do the usual annual stuff, they use
a long applicator wand to spray a fine mist of the material throughout the
interior metal surfaces of the airplane. It flows out and penetrates into
seams, fasteners, etc.
There is one downside of this treatment: the fluid will weep out of seams in
the airplane for several months and attract dirt, requiring additional
washing.

Not to mention the increasing weight and change of W/B each application.