How to clean the interior of a microwave?

My sister burned some plastic in her microwave. It has a typical
off-white painted, enameled?, metal interior, and it's gotten
surprisingly dirty. Spray cleaners don't do much, I guess because
they are meant for grease. (I cleaned the glass tray with a
razor-blade scraper, but even with that, it was hard to get everything
off.)

Soft-scrub products and iirc Bon-Ami use calcium-carbonate, which has
a Moh hardness of 3. While I've used but never visibly scratched
anything with either of those products, I don't think I've used them
on painted surfaces. Especially in a hard to reach inside, normally
just wiped microwave, scratches will pick up dirt and look terrible
(if I bend over!). Even the Soft-scrub surface guide doesn't seem to
recommend this**.

Talc has a hardness of 1. I have some of that, and some baby powder,
somewhere and plan to try it, but ....

I see that gypsum has a Moh rating of 2. Can I make some cleanser out
of a piece of sheet-rock? Pound it down to dust (wearing a
respirator?), add a little water, and some kind of thickener??? This
is a lot of work for one microwave, but it may be useful many other
places. Maybe I could market it and get rich. OTOH, there are people
who do nothing all day but work on new cleansers, and this is pretty
obvious, so they must have tried this already. Still, maybe it is
better for this particular purpose but not marketable enough that they
produce it.

A) How should I make gypsum-based cleanser?
B) What do you think of the whole idea?
C) Do they already sell this and I've missed it?
D) Comments?


** http://www.softscrub.com/guides/surfaces?surfaces=8

It recommends the product for enamel, but there is vitreous enamel
(fired in a kiln) and enamel paint (which is also heated sometimes --
what is the difference?), and the first is a lot harder than the
second. The Softscrub table has no entry for paint.
--
Posters should say where they live, and for which area
they are asking questions. I have lived in
Western Pa. 10 years
Indianapolis 7 years
Chicago 6 years
Brooklyn, NY 12 years
Baltimore 27 years