I have a persistent rust problem on the lower edges of rear fenders
where the wheel well liner blocks access to the back side. The lower
edge seam wasn't watertight and rust has perforated the outer and
wheel well liner sheet metal below the factory sealer bead, which was
applied from inside after spot-welding and didn't reach down to the
joint. I dug out most of it so I could MIG weld patches without
igniting it.

Several local paint suppliers have suggested 3M 08852 Cavity Wax Plus
to coat the inside of my larger welded and smaller resin patches to
prevent or slow the so far yearly rust lines along their edges. LPS-3
wasn't up to the task, or I didn't coat the out-of-sight area well
enough with it. The Cavity Wax comes with long applicator tubes.

I'd like advice on if it really works, and how difficult it is to
remove if for some reason it doesn't and I need to weld in a larger
patch.

The clerk told me it's better than rubberized undercoating in the
wheel well. The suggested alternative there is spray can truck
bedliner, assuming it can be removed if necessary.
tia
-jsw