Window gasket cleaning

I bought a GT rear window gasket a few years back from someone who sold Opel parts at that time in the North East. He had an "all sales are final" policy.

The gasket has white gunk on it which covers all surfaces and is caked on the inside surfaces of the gasket too.This will only ruin chances of a watertight seal. Do all new gaskets come this way or is this just a screw up? How do you clean this stuff off? It sticks to the gasket like glue and is whitish and kind of like a scum. It has to be rubbed or scraped off and it takes a very long time to do any particular inch of gasket. Was this a mold release or a preservative? I can't imagine any body shop accepting this crap as fit to install without hours of cleaning (which they would never want to do). Is there a solvent that works and speeds things up?

Once cleaned.....how necessary is it to use a sealant between gasket and body or gasket and glass? I live in the Midwest. The metal surfaces at the lip of the body are excellent, painted with no rust...just some remainders of some super silicone that must have been smuggled out of Nasa which I will try to remove as it will also just interfere with a good, smooth, watertight seal.