Most of your voltage drop is probably occurring where parts come together. Metal, all metal to some extent, oxidizes and metallic oxides are worse conductors that the metals. So the first step is tho clean ALL the places where electricity has to go from one part to another.
The sticky stuff on the back of the copper is probably a poor conductor.
Noalox is a grease that will help keep out oxygen to slow oxidation and it contains Zinc particles that will cut through surface oxidation and improve conductivity. If you do use it use just a tiny bit, It will attract dirt so clean often, only put it in places where the air you inhale does not go over it.