I just picked up an RX in fine working condition that I wanted to clean-up a bit--there were some spots on the platter.  Foolishly not researching beforehand, I lightly used some Cameo aluminum and stainless steel polish, cleaning after with soapy water then just water.  That unfortunately didn't polish but instead slightly dulled a portion.  Then I tried a little isopropyl alcohol, cleaning after with soapy water then just water.  That seemed to restore some shine, but also left some whiteish residue at the edges. UGH!!

So...

1) What can be done now?

2) Is the platter aluminum or stainless steel?

3) Does the platter come off easily and without removing the belt (I'm assuming is is belt driven)?

The platter will be anodised aluminium. You have removed some of the anodised surface. Not sure there is much you can do about this. Probably the best is to leave well alone but if really nasty, you could spray and lacquer. It won't look original though. The platter just lifts off - a palette knife can be used as the fit to central section is quite tight. Just don't scratch anything! There is a sub-platter which is belt driven.

I had the same problem, I did as follows:

Get Scotch Brite pads,

http://www.instawares.com/scotch-brite-light-duty.mco98.0.7.htm?LID=IWAMZ&ci_src=23393768&ci_sku=MCO98

Cut a small square 25 x 25 Mm, put the platter on a flat surface, kitchen table, your wife will love this, to the pad ad some liquid soap, normal housecleaning soap will do, now rub hard following the polished circular pattern on the plate, keep wet with the soap, when the pad is medium gray on the used side, turn around, when bouth sides is used take a new one, finishing the outer part, clean with hot water, not boiling hot, use a dish brush to get of all residius. Dry check if not nearly clean go one more time, there may be some small differences in the surface, but thats OK, do the rest same way.

Now get some car polish, medium, or fine, I used medium. In a small bowl mix a little water with polish, cut a small piece of your T-shirt, dip in the mixture and rub as before, keep it wet, clean with dish brush, hot water and dish soap. Dry with towel and let dry totally, check if you like it its OK.

When the platter looked OK I took a small amount of Silicone grease/oil lubricant and with a small pad of folded kitchen roll I rubbed lugricant into the platter, and at last rubbed with clean paper untill paper came of clean. All in all some 1.5 hours work, and let your wife clean the kitchen.

Here the result, the black paint missing was missing before I started.