cabinet shielding question

[Randall]

I ended up sanding the four corners of the chassis lip (the highest edges), and stapling down the flashing with many staples. Then I removed the handle and put one probe on the chassis, and the other thru the hole touching the flashing. Nothing. Until I scratched it with the probe a bit, then I got 0.3 ohms. This then led me to find out that the coating on the flashing is non conductive, at least to a point. PLacing probes on a fresh new piece gets me nothing when placed 'sideways", or the shaft of the probe. Only when I poke or scratch it with the probe points does it conduct. The galvanizing, or whatever it is seems pretty thin, so maybe one or more of the chassis corners scratched it enough to make the connection.

Perhaps I need to scuff up the flashing a bit in those corners.

[Chuck H]

I ended up sanding the four corners of the chassis lip (the highest edges), and stapling down the flashing with many staples. Then I removed the handle and put one probe on the chassis, and the other thru the hole touching the flashing. Nothing. Until I scratched it with the probe a bit, then I got 0.3 ohms. This then led me to find out that the coating on the flashing is non conductive, at least to a point. PLacing probes on a fresh new piece gets me nothing when placed 'sideways", or the shaft of the probe. Only when I poke or scratch it with the probe points does it conduct. The galvanizing, or whatever it is seems pretty thin, so maybe one or more of the chassis corners scratched it enough to make the connection.

Perhaps I need to scuff up the flashing a bit in those corners.

The flashing I have is just plain aluminum sheet. It's (probably) coated with some kind of very light finish to keep it from oxidizing before they get it out of the store. Aluminum oxide is a decent insulator on it's own.

I use HD aluminum foil and rubber cement for cabinet shielding. I haven't had any trouble with oxides keeping it from connecting electrically to the chassis.

[Helmholtz]

Aluminum has self-passivating properties. Meaning that it covers itself with a very thin but dense and hard (insulating) oxide layer. This protects the ignoble (base) metal from further corrosion. Good electrical contact typically requires some minor scratching or piercing.

[J M Fahey]

I use HD aluminum foil and rubber cement for cabinet shielding. I haven't had any trouble with oxides keeping it from connecting electrically to the chassis.

Same here.
I contact cement a sheet of kitchen grade aluminum foil to cabinet (dollar shop was momentarily out of NASA grade ), then make a small peanut sized ball of same aluminum, wrap 1 or 2 turns of flexible wire around it, peeling about 1 inch first,of course, and *staple* that soft terminal against the aluminum "roof".
I solder the other end of the 10/12 inch wire to supply ground, so I can partially remove chassis if needed.
I do NOT depend on painting/unpainting, screw heads making contact, anything mechanical which can (will) eventually come loose, but a solid *soldered* to ground wire.

Why the crushed aluminum ball?

If I straight staple thin kitchen foil, sometimes holes enlarge and it does not make good contact any more; the crushed ball provides relatively huge contact surface and is relatively flexible, you NEVER lose contact.