New (to me) Zenith H503Y portable radio.

[Phil Nelson]

That's a cutie.

If the new diode is connected in parallel with the old selenium rectifier, I would disconnect one of the rectifier leads to take it completely out of the circuit.

What I use on TransOceanic cabinets with this "black stag" type of covering is liquid black shoe dye (not polish). The little foam applicator makes it easy to put on, and it may penetrate worn spots better than polish. You will end up with a very even color. You can apply a coat of black shoe polish after that, but I have never found that necessary.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html