O : Without a microscope its hard to make a good guess. My suspicions are some sort of "Last or Sound Guard-type" product or a pressing issue. If either is correct, the problem maybe beyond the limits of cleaning .
I have never done this but for a last resort,dishwasher liquids (which exact ones are the best I do not know) are said to be chocked full of man-made enzymes. Again,I do not know the ratio but a very small amount of dishwashing liquid to warm H2O,shake until disolved,then paint the liquid on the LP,let sit for 2 or 3 minutes,scrub,steam-rinse and a water-bath rinse. Dry w/ microcloth, air dry or RCM. Oilmanmojo, I haven't tried it but it "sounds" as if it may work on some grime but what you encountered I don't know for certian. I think the pressing may be at fault.
I have never done this but for a last resort,dishwasher liquids (which exact ones are the best I do not know) are said to be chocked full of man-made enzymes. Again,I do not know the ratio but a very small amount of dishwashing liquid to warm H2O,shake until disolved,then paint the liquid on the LP,let sit for 2 or 3 minutes,scrub,steam-rinse and a water-bath rinse. Dry w/ microcloth, air dry or RCM. Oilmanmojo, I haven't tried it but it "sounds" as if it may work on some grime but what you encountered I don't know for certian. I think the pressing may be at fault.