You want metalworkers tinning compound, not electronics grade tinning
compound. Metalworkers tinning compound is a mixture of lead dust, and
acid. You apply it with a paint brush, to clean metal, than heat it with
a propane torch . The lead dust forms solder balls, at this point, wipe
area with damp rag, and a thin layer of solder will remain. Reheat this
solder, apply quick wipe of plumbers paste [ flux ] and add some
plumbers solder, which melts at a lower temp than tinning compound.
When sanded smooth, you will have the ultimate place to sink a
ground screw into. Your problem is corroded contacts in snap connectors ?
Your better off finding similar connectors at the junkyard and
change out your bogus ones one at a time. Failing that, get a dental
pick and gently scratch each connector. A FiberGlass Bristle Brush
is available for shining up rust nicks in car bodies, called Rust
eraser, its plenty abrasive, while short lived, does wonders on
mildly corroded contacts. Good luck. One last tip ! Coat the contacts
with water mix valve grinding compound, snap them together, and apart a
few times, than brush with toothbrush [ the contacts ! ] Use water mix
because its easy to wash away.