If I remember correctly, it has to do with the wavelength of the
microwaves. Put a big piece of metal in the microwave and it probably won't
do anything. Put two pieces of metal in close proximity and the microwaves
will tend to spark between them. This includes items placed too close to
the walls of the microwave.
Lay a piece of aluminum foil out flat, it probably won't spark. Crumple it
up into a loose ball, and you'll probably get sparks between adjoining
Stick a paper clip in the microwave, it will probably arc. Straighten it
out and it probably won't arc.
Plates with gold rings around the outer edge tend to spark because the ring
isn't electrically continuous. It may look like a solid ring, but there are
microscopic gaps that allow the microwaves to arc.
It's all about the distances between pieces of metal.
You can use normal paint, as the oven doesn't get any hotter than the food
you are cooking. Be sure to remove the rust, sand it lightly, and wipe it
clean with laquer thinner or something before spraying the paint on. You
may want to use an appliance paint for longevity. I just bought some from
Home Depot last night for another project.
Obviously, wait until the paint is dry before you start cooking with the
oven. I would probably boil a cup of water a couple of times and discard
it before I started putting my food in there.
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