Javier,

I highly recommend, staying away from the Scotch Brite pad, or
sandpaper, unless of course your ready to refinish the top, in your
cleaning frenzy----

You will inevitably will remove whatever finish there is left  of the
jell-coat, and leave the surface of the poptop open,        (scratched)
causing you mucho grief----

If you want the grey out of the top---get ready------ rent, borrow an
electric buffer, along with some 3-M super heavy duty rubbing compound,
and buff the hell out of it, finishing it off with a couple coats of
premium wax.

This sounds like that this is going to be your only solution to removing
the oxidation.

Have Fun-----

            ______________
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Terry--
74 Campmobile- (Clementine)
85 GL- (Delilah)
86 BMW 325 ES- (Eva)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~     "In life, it is much better to want
something you cannot have---

Than to have something you can't get rid of"

(T.K.1979)