How can one clean the whitish gray "bloom" from vintage eyeglass
frames?
 I wear vintage eyeglass frames(I have several pairs) and over
time they tend to develop a white substance on the surface that can be
scraped off with a fingernail. An employee(not the optometrist ) at an
eyeglass shop told me that the "bloom" (it's not unlike the whitish
color on the surface of a concord grape) is caused by resins rising to
the surface of the plastics the old frames are made from, and I should
rub the frames with hand lotion to replace the resins and leave them
in a Ziploc bag overnight. Well, that didn't work. 
What I want to know is if there is a way to clean the substance off
without scraping it off with your fingernails. (too time
consuming/difficult to get all of it out of every nook and cranny)
I've tried plastic scrub pads, which don't seem to work very well
--not abrasive enough--and a way to prevent it coming back once it's
been removed.
 If there is a type of cleaning product that would work well, I'd like to
know. I don't clean my glasses with anything but water and a soft cloth.

I would try a non-metalic scouring pad.

I would first try mineral spirits.  If that doesn't work, try the Mr.
Clean Magic Eraser.  Lastly I would use automotive polishing compound
(available at any auto parts store).  Be sure to get polishing
compound, not rubbing compound which has a coarser abrasive.  As with
all cleaning methods, try it in an inconspicuous spot first to make
sure there are no adverse effects.